I don't quite know how to detemine what key a song is in, whether it is chord progressions or a solo.
Well in a non technical, theory jibberish way you can work it out by trying to work out the first riff with simple power chords or sumthin and usualy the first note in the song is the key of the song.

mmmmm.... i duno
If it's done on a stave then there should be a little place after the treble clef (in the case of guitar music) which is where the key signature resides. The number of sharps or flats indicates the key. For instance, one sharp = G major, if there's nothing there at all then it's in the key of C major. That's because G Major has one sharp in it and C major has no sharps or flats. And of course you could also play in the relative minor key, but that's not what you're asking.

I think that's it basically.