If you want to assure someone that you're not something, try saying that you used to be one. This may just be enough to blindside them into believing you aren't any more.
"Are you a thief?" "No, but I used to be one. Retired now, don't you know?"
"Are you a masked superhero?!" "No, but in my younger days I was the Scarlet Sparrow. Gave it all away after finally defeating The Jester for good."
If you really want to establish this before suspicion falls on you, bring it out up front!
"I know we're all looking for the vampire. I just want to say I used to be one, but I got better. Just putting that out there, so you all know."
Commentary by Keybounce (who has not seen the movie)
[Keybounce's comments will appear here when received.]
Commentary by memnarch (who has not seen the movie)
Bwahahaha! What the heck happened in this scene!? That face! It's so great! More crazy Harrison Ford faces please! But seriously, I can't think of anything that would have this kind of interaction between Finn and Han in the movie. Han looks like he's both annoyed and being bothered by Finn and is totally messing with him at the same time. And then Finn is left completely speechless holding a giant gun.
Also, nice to know that Yanni didn't sell out the good guy side any further after getting caught out. It would have been rather awkward for everyone to run into that situation again. Though perhaps that's because people were expecting it to happen, so there wasn't any more money to have in playing double agent.
Transcript
{Inside the Falcon}
Finn: So, Yanni—
Yanni: Are you some sort of double agent for the First Order?
Finn: What? No!
Yanni: I saw you pretending to be all casual, asking questions about what the Resistance knows.
Finn: I’m... just genuinely curious.
Yanni: Relax, I know all about being a traitor. Used to be one myself.
Yanni: Five times.
Yanni: Here, take this. {hands Finn a blaster}
Finn: What for?
Yanni: Watch your back, kid. You don’t want to be shot by another traitor.
Yanni: Whoever that might be...
{beat, Finn ponders as Yanni walks away}
{beat}