Characters can use the Create Advantage action to discover another character's power facts by discovering the aspect which provides them. This applies to both character aspects ("he's using telekinesis") and situation aspects ("it looks like there's a force field here, and it blocks radiation").
This is only relevant in encounters where a power's true nature is intended to be important to the adventure, such as when fighting an invulnerable villain with a secret weakness or investigating a mysterious alien artifact. Characters who can fly, throw bolts of fire from their hands, and the like, usually have obvious power facts.