Have them roll Empathy for people who aren't doing anything nefarious, just to get an impression of their personalities.
Even that's somewhat flawed, though. FATE in general doesn't do this stuff very well. The FP spending rules tend to assume a certain level of transparency.
That's a good idea, but it's probably undermined by the fact I haven't really had them do that up to now. And yeah, the fate point spending makes it tough to sneak things by.
Put another NPC in the party that doesn't seem "on the level" that actually is; he's just really not a likeable guy. Make sure he's involved in whatever discussions the mole is, and players will immediately think they're rolling empathy on the "smelly" NPC companion. When they finally find out he's a decent guy, it'll be egg on their faces for using the OOC info that they were rolling Empathy.
If/when they ask what they're rolling for (to determine if they want to boost with FPs) simply reply "you are rolling Empathy to attempt to uncover someone's hidden motives in this conversation," or something equally descriptive while not giving something away.
I wish I had asked this sooner, cuz that's a good one too. Unfortunately, it's too late to change the parameters of the start of the scenario.
One of the other things you could do is simply compare their empathy scores to his deceit score (as if both parties had rolled zero). If they are close then you can give them some hints ("He seems like he's holding something back" or "You aren't sure he's quite on the level").
That might be the simplest way--looking at their sheets, the Mole's deceit is 5 (up to 7 with the Honest Lies stunt), and the highest Empathy a PC has is 3.
Just to be clear do you want them to uncover the mole, or do you just want to give them the chance before things go to hell?
The way the Scenario's laid out in my mind, I'd rather they didn't, but I want them to have the chance to.
Either way some foreshadowing (related to the player's actions, not specifically to the NPC) might make the players paranoid enough to begin questioning everyone.
Hm, I could push that angle. Part of the scenario is a case of mistaken identity meant to make a newly-introduced PC seem like the badguy for a bit to one of the other PCs.
@Richard: Yeah, I was planning to have him tell
mostly the truth (partly because when I initially planned the scenario, the party included a dog who could discern lies--hence the Honest Lies stunt, he was meant to lean on that a
lot). Though he's newly introduced to the main group of PCs, so he hasn't had time to develop that kind of "always in the background" alibi, for them at least.
One thing that popped into my head was just rolling for the PCs without their knowledge--but that feels a little wrong, not least because they wouldn't get the chance to spend fate points to make the rolls without me telling them what was going on.