How do I get an alien to assume they're alien?

[monetsalien]

Ok so I threw a party to my sim since it was her birthday and then I noticed that one of the guests had a glowing green aura, at first I thought it was something from the dj thing, the lasers and whatnot, but then she changed the room and the glow was still there. Now I know this is clearly an alien thing but I found no interaction that could make her reveal she is indeed an alien.

[MarianT]

Welcome to the Forum, monetsalien!

Unless your sim is working as a doctor, detective, or scientist, there's no way that a non-alien can find out if another sim is an alien. But if they talk to each other long enough, the alien might try to empathize with your sim or analyze her personality, which will make her realize that she's dealing with an alien.

[Playalot]

You can often discover an alien when woohooing with them or as MarianT said, and through conversation especially the 'deep conversation' interaction although just using friendly socials normally works.

When a disguised alien is in any extreme emotion (e.g. very happy, very Energized) they will emit a glow, this is a telltale sign that this 'normal' looking sim is an alien in disguise.

[Nimrod Mikkusu]

Often if I have an alien and a non-alien sim in the same household, when they talk, the non-alien sim discovers that the alien is an alien, too.

[Saltypaws]

Often if I have an alien and a non-alien sim in the same household, when they talk, the non-alien sim discovers that the alien is an alien, too.

This is true, had a sim go to the bar on alien night and this is exactly what happened.  Pretty cool.