Quote Originally Posted by Joe the Rat View Post
Seconding on this - it's detecting the motive. Note you should also look at this initially - passive insight, unless someone questions the motive.

My other thought is Perception vs Sleight of Hand (concealing the move), but that's basically one Dex check or another.
That one could go either way. In Real Life, if I see that you have a knife out and you're coming at me, I'm ready to move out of the way. I do that even when it's a family member--I know they're not going to poke me intentionally but I don't trust them not to stumble over the edge of the carpet.

So, I'd say in general, the assassin would have to win a Deception v Insight for me to think "Okay, he wants to be my ally" when I first meet him and occasionally afterward. At the time of attack, probably a Sleight or Stealth vs my Perception to get a Sneak Attack in, although I might well have Disadvantage if I'm distracted, or no check at all if I'm very distracted or asleep.

Note for the DM: Don't be so in love with this idea that you ignore your players and force it to happen. I've had a DM do this to a party I was in, giving the traitor complete surprise despite my character saying for three weeks that we don't know what this dude is up to, we shouldn't trust him, we need to keep an eye on him, never let him stand watch alone, and such.