Sus.

how could you..

suspicious
/səˈspɪʃəs/

adjective
having or showing a cautious distrust of someone or something.
"he was suspicious of her motives"

doubtful
unsure
dubious
wary
chary
sceptical
distrustful
mistrustful
disbelieving
having reservations
apprehensive
cynical
jaundiced
iffy


trustful
trusting
causing one to have the idea or impression that someone or something is questionable, dishonest, or dangerous.
"they are not treating the fire as suspicious"

disreputable
unsavoury
dubious
suspect
guilty-looking
dishonest-looking
strange-looking
queer-looking
funny-looking
slippery
shifty
shady
dodgy
questionable
odd
strange
irregular
queer
funny
doubtful
not quite right
under suspicion
mysterious
murky
dark
criminal
dishonest
corrupt
fishy

upright
ordinary
innocent
having the belief or impression that someone is involved in an illegal or dishonest activity.
"police were called when staff became suspicious"

when the imposter is acting mildly apprehensive

When the faker is performing actions in a very questionable way, causing you to have a thought in your mind that they are "sus".

at the point when the impersonator is acting rather untrustworthy