Shapeshifting and illusory powers are important to watch out for, as well. Shapeshifting powers are usually limited in scope, and as such, will typically be used to impersonate someone of a similar height and weight to the imposter, possibly with similar facial features, etc. However, some shapeshifting powers may be extremely dramatic in presentation, especially those that allow someone to transform into e.g. animals, and can be grouped together with illusory powers, which typically have little limit with regards to a field the size of a human.
The easiest way to trap imposters in such scenarios is by asking for identification, keys and passes, etc. Although certain illusory powers may be able to mimic IDs even when out of the imposter’s grasp (and many will have access to manufacturers of fake IDs), they should fail to validate as genuine on standard equipment. (Reminder to run identification, verify passes, etc. under the screening ledge, as so many illusory powers work on line of sight. In case of suspicion, IDs may be rerun in a different room, to avoid proximity and other typical constraints of illusion.) Keys and passes would likewise not be identified as real. Asking security questions or mentioning any pieces of personal information are also effective ways to trip up imposters whose powers work in similar ways. Even when they have access to some personal information, it typically remains insufficient to answer targeted questions.
Many illusionists can see through illusion powers, as well as certain others with powers that reveal, etc. Many espers can recognize shapeshifted individuals, although they may not recognize illusions (depending on what senses the illusion covers). In the case that there is no corresponding extrasensory illusion, this will obviously be quite simple to spot, but in many other cases, multiple senses may not correspond as well as usual (this can sometimes be noted by those with no enhanced senses). Body language and other kinematic experts may be able to recognize either illusion or shapeshifting, as the observable body shape will likely not be one the imposter is overly familiar with. As typical, memetics, mirrors, acutes, and perceptives are likely to root them out.
Another common method is the use of telepathic or reality warping powers to give the impression that some individual should be there or always has been there. This can be used to mimic a specific person, but is more likely to be used to overwrite a new individual into the daily workings of the WHC. Depending on the strength or type of power, this may be very long-term (i.e. the person has been working in the building for years) or situated as more of a recent development (e.g. a new transfer). Telepathic powers are more likely to be limited in scope, and may have to be aimed specifically, but broad-spectrum telepathy is not unheard of, and may in any case be confused with reality warping, which also affects all targets within a given area. Some reality-warping is limited to a small area, touch-contact, etc. and may affect only a very limited subset of the agency.
If someone outside your department says this is happening, follow all protocols immediately. (If it turns out to be a prank, report them; it is probably not a prank.) Otherwise, suspicion of these imposters may be unlikely or difficult to come by, although entertaining the suspicions can itself be a helpful act, as it may take more energy for the imposter to maintain the altered version of events. With telepathy, artifacts are unlikely to be affected, so rosters, photographs, signatures on reports, etc. should reflect the nonexistence of the imposter, and when encountering such evidence, one should be careful not to write it off. With reality warping, this is more hit or miss, but inconsistencies of memory, such as gaps, conflicting memories, multiple memories of the same event, or memories inconsistent with factual data can indicate that something is going on. In all cases, be very aware of warnings or even simple confusion on the part of members of other agencies, other branches, or especially coworkers on vacation, sick, or working remotely. These may be your first or even only indicators.
Those with enhanced, especially perfect, recall will often be more aware of the possibility of reality warping, although may be uncertain if on the periphery of changed events. Telepaths and some other memetics are more likely to be immune to telepathic intrusion. Those with mental training may have some resistance to both types, and dogs and horses will generally respond negatively to the imposter (and may be available for a quick look – preferentially ask for the dogs). Give the benefit of the doubt to those individuals who are Earth-baseline grounded; they will almost always be unaffected by such events. Information can be double-checked with baseline grounded equipment. As typical, memetics, mirrors, acutes, and perceptives are likely to root them out. In this case, especially give the benefit of the doubt to any and all nulls.
A less likely, but possible, scenario, is that a telepath has somehow gained access to the mind of an actual agent or other member of staff. This may be through violence or intimidation, blackmail, etc., but not all telepaths reach their targets through their weaknesses or vices. The telepath may pose as a friend or even just an acquaintance, and slowly work their way into someone’s life. This may result in someone with enhanced knowledge of current cases and the inner workings of the WHC, using any of the typical infiltration methods. It may also result in someone modifying, putting pressure on, or controlling someone’s mind as they go about their work, in which case the correct person will be there, but behaving differently.
Someone mimicking a member of staff may have plenty of knowledge, including personal knowledge, but will likely be unskilled at applying it. They may be seen fumbling at or botching everyday tasks. They are also likely to display different personality traits than the person they mimic; report changes in behavior and activities to patterns. In the other cases, the staff member may be confused or disoriented, anxious, or distant. (Again: patterns.) Note that not all cases of stress are indicative of imposter incidents, but do watch out for sudden onset of stress, especially in individuals where it’s rarely present. Engaging individuals in critical thinking games is likely to help in most incidents.
Imposters can be picked out the same way as an imposter without access to personal information, although possibly not as effectively, as well as typically being quite obvious to telepaths in their mind/body mismatch (or multiple minds, etc.), but may be more apparent to those with close bonds to the persons being mimicked, as the imposter may well try to be overly friendly. Modified memories should be clear to anyone who shares the unmodified versions, as well as to anyone with memory- or thought-pooling abilities. Those having their minds influenced are likely to be noted by the psychological staff, or anyone familiar with symptoms of addiction (these can be distinguished easily, because the onset will typically be quite sudden in the case of telepathic manipulation). Those with meditation or coercion-neutralizing powers may be able to help in these cases. Mind control is likely to be noted by anyone with numerical or distance-measuring powers. As typical, memetics, mirrors, acutes, and perceptives are likely to root them out.
Keep in mind, technopaths are likely able to create false identities in ways beyond the scope of measure of the average false identity. Their paper trails may be vast and elaborate. However, the typical technopath has absolutely no ability to create false memories, and may well be remembered specifically and negatively, as spontaneous technopaths tend to be highly anti-social. They are virtually always incapable of the level of charm or even friendliness necessary to lay the groundwork for this type of event, and so don’t need to be seriously considered unless one is already under suspicion. If worried about a potential technopath, call their last post to verify employment history, and speak at length to their references, in person if at all possible. Technopaths looking to join the agency for the first time may be harder to vet, and as such, it’s recommended they be evaluated in more depth, although the application process does weed out most imposters. And, as typical, memetics, mirrors, acutes, and perceptives are likely to root them out.