Gambling
Defaults: IQ-5 or Mathematics (Statistics)-5.
This is skill at playing games of chance. A successful Gambling roll can (among other things) tell you if a game is rigged, identify a fellow gambler in a group of strangers, or "estimate the odds" in any tricky situation. When you gamble against the house, make a skill roll (the GM will secretly modify this roll if the odds are poor!). When you gamble against someone else, roll a Regular Contest of Gambling until one of you wins.
Sleight of Hand skill is helpful if you want to cheat! To spot a cheater, roll a Quick Contest of your Gambling or Vision roll, whichever is higher, vs. your opponent's Sleight of Hand skill (for card or dice tricks) or IQ (for other kinds of cheating).
Modifiers: +1 to +5 for familiarity with the game being played; -1 to -5 if the game is rigged against you; -3 for Killjoy, since you don't care if you win or lose.