This is an interesting question but it may not have a definite answer.
You might recall that when Jesus cast a legion of demons out of the Gadarene man, they pleaded to enter pigs (Mk 5). Some have taken this to imply that demons are unable to assume corporeal forms of their own and are therefore restless to inhabit the bodies of other creatures. Certain scholars have even argued that God's curse on the fallen angels includes taking away their to manifest bodily, in the way good angels do throughout the Bible. I must admit, however, I don't know of any clear Scripture which confirms that theory. Moreover, if the demons were so anxious to escape their disembodied state, why did they immediately drive the pigs off a cliff? I think the legion might simply have been overcome with an insatiably desire to possess and harm God’s creatures, regardless of their ability to manifest corporeally.
Elsewhere, Paul warns the Galatians not to receive any false "gospel" even if it is presented by an angel. Since no good angel would spread a false gospel, it seems like demons can literally pose as angels of light. But Paul might simply be exaggerating for effect. Something like exclaiming today to the guards as Buckingham Palace, “even if King Arthur comes to you, don’t open these gates!” It doesn’t have to be based on a real possibility to stress the importance of one’s duty.
Compare also 2 Corinthians 11:14, which says "Satan disguises himself as an angel of light." This suggests he can masquerade in other forms. But it's possible Paul means this only in a spiritual way, which says nothing of Satan’s ability to appear in human form.
Whether or not demons have the ability to appear like humans, I think one thing is definite. The Bible presents no clear evidence that demonic impostors are something we should expect or specially beware. If you meet a very bad man, go ahead and think, "that's a bad man," not, "he might be a demon." Personally, if necessary, I’d call the police and not an exorcist to make sure.
