For example... Dave is using intimidate. What skill should the enemy use to oppose it?
But not just intimidate. I'm just curious if there's a quick place to figure out what is the best opposing skill when it isn't obvious.
It's pretty much context-dependent and up to the GM in most cases where it's not spelled out by the book. There's no table of hard-and-fast "skill X is opposed by skill Y".
Look at each skill's description. For skills that have opposed rolls, it will usually say what opposes it in the text. If, however, it just says something about opposing a defense roll, then you have to look at which skills can be used as defense, according to their skill entry.
Using Intimidate as an example, it depends on what trapping you're using. The "Brush Off" trapping explicitly says "you may roll a quick contest of Intimidation against the opponent’s Discipline or Presence." On the other hand, the "Social Attacks" trapping says "The target usually gets his choice of social skill to defend against such an attack—for whatever help that might ofer him—so long as it’s appropriate to the attack."
So, actually, based on that, there are even situations where multiple skills are appropriate to oppose something, and either the GM or the Player will get to choose which one to use.
