you don't. you tell what power chords are in a key.

a key is relative. whatever note you choose as the root is your key. Simply put, you choose what scale/key you are going to use from your first note, and the type of scale will determine the sharps or flats, and the major or minor quality of the chords.
a power chord is a root and a perfect fifth....theres no way to tell the key by looking at one power chord, besides assuming that the root note of the chord is the key. Now if you have a succession of power chords, think of them as single notes (root note) and then match it to a major or minor scale.