In Fate Core i know that you can invoke the aspect of your enemies against them like making someone who has Trouble Controlling Strength get the Aspect Overextended by invoking it and saying due to swinging too hard they lose their balance.
What i want to know about is how does the mechanics of identifying enemy aspects work? Do you need to roll empathy or is it done over time? Do you get a free invoke of that aspect when you discover it?
In Fate Core, just run out of the box? You need to roll Empathy and it's done over time.
Create an Advantage: You can use Empathy to read a person’s emotional state and get a general sense of who they are, presuming you have some kind of interpersonal contact with them. Most often, you’ll use this to assess the aspects on another character’s sheet, but sometimes you’ll also be able to create new aspects, especially on NPCs. If the target has some reason to be aware that you’re trying to read them, they can defend with Deceive or Rapport. (from here)
Like all uses of Create an Advantage, you get a boost on a tie, and one or two free invokes on the Aspect on a success or success with style, respectively. Exactly what constitutes a reasonable amount of interpersonal contact to make the roll is up to you and your GM to work out, but it's probably going to be more than a single combat exchange.
However, stunts can change that. One of the stunts on Notice, for example, will let you roll that instead, with the same general limits on contact and time. It wouldn't be unreasonable for someone to take a stunt on Fight to make the same roll, with the "interpersonal contact" being a fight you have with them, win, lose, or concede.
Turning Trouble Controlling Strength into Overextended is up to you and your GM exactly how that plays out. If you just spend a point and it happens, that's invoking for effect and I see no reason not to let you spend the free invokes you got with Empathy, but usually invoking for effect happens at the speed of plot and creates more narrative consequences than just adding another Aspect.
It may be more likely that you or someone else Create an Advantage during a combat exchange to place the Overextended Aspect on them, probably with Provoke, since Trouble Controlling Strength is part of what they are and Overextended is just what they have right now. You can translate the Empathy free invokes into greater effect on that roll, and also, since Aspects are true, they're not going to have a good defense against being lured into going Overextended.
I mean, I hope the GM didn't make somebody with Trouble Controlling Strength and also like Will +6 or whatever.
