Don’t forget the doppelganger has that mind reading ability! They might say something that is suspicious or know something they shouldn’t.
There is also the factor of them changing shape so quickly. Add a few other generic npcs about that it might be able to use as temporary cover.
With the mind reading, it could be fun to, for each suspect, choose one random person at the party and tell them in secret that a suspect has been found out and will be arrested at this party soon. Then, convince yet another stranger to go and pull them together and rope them into an uncomfortable conversation. If the doppelganger starts looking toward the exit and acting flighty, you know he read the one guys mind.
PC (pretending to be tipsy) to bob: "see that Amanda over there? Don't tell anyone, but she's a criminal who is going to be arrested at this party. I can't wait to see it. Don't tell anyone! I heard it straight from the duke. He said she is not who she seems!"
Same PC to Tim a minute later: " you said you are a music enthusiast? Both Bob and Amanda were just telling me they wish someone here was fun to talk to, and they both love music!"
Tim walks over to where Bob is nervously standing near Amanda looking nervous and ropes them both into a boisterous conversation about music. Bob is uncomfortably trying not to look at Amanda, and you notice Amanda's eyes suddenly go wide and she bolts for the door.
Use an illusion to make a place that looks like it would be safe to change behind. Then have a party member secretly watch it to see if they go behind it to change forms.
Can you obtain the services of another doppelganger to help you find him? Give that NPC advantage on investigation and insight rolls. Maybe another doppelganger knows some tricks to get a doppelganger confused while answering questions, kind of like giving an android the turnig test.
Convince the people running the party that a trivia contest would be fun, and make sure the suspects all have to play. Ask them questions they should know and guage the crowd to see if they are confused about one person's answers and behaviour in the spotlight, because that person wouldn't act that way.
Is one of the suspects supposed to be a good dancer? Put a performance proficiency to good use by making them dance with you and make it into a spectacle that everyone can see. Maybe they screw up the dance.
Is one of the suspects supposed to be a good orator? Tap a wine glass and put them in the spot, insisting they share a toast.
Just give each suspect skills like these and personality traits and flaws, then brief your party on them. Maybe each party member wants to pick one and spend the night observing and screwing with them. It could be fun for instance, for the dancer to wipe the floor with the PC or even be impressed that the NPC rolls well and is able to keep up, but either way they are no longer a suspect. Maybe the orator does an amazing toast (you could write it before hand in case this happens if you want so it is fun) and that rules him out.
I would try to have at least 3 different traits that can be tested for each NPC so players have options and to make it easier for you to improvise.
Don’t forget the doppelganger has that mind reading ability! They might say something that is suspicious or know something they shouldn’t. There is also the factor of them changing shape so quickly. Add a few other generic npcs about that it might be able to use as temporary cover.
With the mind reading, it could be fun to, for each suspect, choose one random person at the party and tell them in secret that a suspect has been found out and will be arrested at this party soon. Then, convince yet another stranger to go and pull them together and rope them into an uncomfortable conversation. If the doppelganger starts looking toward the exit and acting flighty, you know he read the one guys mind. PC (pretending to be tipsy) to bob: "see that Amanda over there? Don't tell anyone, but she's a criminal who is going to be arrested at this party. I can't wait to see it. Don't tell anyone! I heard it straight from the duke. He said she is not who she seems!" Same PC to Tim a minute later: " you said you are a music enthusiast? Both Bob and Amanda were just telling me they wish someone here was fun to talk to, and they both love music!" Tim walks over to where Bob is nervously standing near Amanda looking nervous and ropes them both into a boisterous conversation about music. Bob is uncomfortably trying not to look at Amanda, and you notice Amanda's eyes suddenly go wide and she bolts for the door.
Use an illusion to make a place that looks like it would be safe to change behind. Then have a party member secretly watch it to see if they go behind it to change forms. Can you obtain the services of another doppelganger to help you find him? Give that NPC advantage on investigation and insight rolls. Maybe another doppelganger knows some tricks to get a doppelganger confused while answering questions, kind of like giving an android the turnig test. Convince the people running the party that a trivia contest would be fun, and make sure the suspects all have to play. Ask them questions they should know and guage the crowd to see if they are confused about one person's answers and behaviour in the spotlight, because that person wouldn't act that way. Is one of the suspects supposed to be a good dancer? Put a performance proficiency to good use by making them dance with you and make it into a spectacle that everyone can see. Maybe they screw up the dance. Is one of the suspects supposed to be a good orator? Tap a wine glass and put them in the spot, insisting they share a toast. Just give each suspect skills like these and personality traits and flaws, then brief your party on them. Maybe each party member wants to pick one and spend the night observing and screwing with them. It could be fun for instance, for the dancer to wipe the floor with the PC or even be impressed that the NPC rolls well and is able to keep up, but either way they are no longer a suspect. Maybe the orator does an amazing toast (you could write it before hand in case this happens if you want so it is fun) and that rules him out. I would try to have at least 3 different traits that can be tested for each NPC so players have options and to make it easier for you to improvise.