If the Key Sig has #s, go up a semitone from the rightmost # (on the score) to get the major key.
i.e. 3 #s, the rightmost one is G#, so the Major key is A. 6 #s, the rightmost one is E# (F) so Major key is F#
If the key Sig has b's, the 2nd b from the right is the Major key. (Note: this doesn't work for FMajor, but that has only 1 flat anyway)
i.e. 2 flats, the 2nd rightmost flat is Bb, so the Major key is Bb. 5 flats, 2nd rightmost flat is Db, so Major key is Db.
EDIT:
Note that I emphasize that is an indicator of the MAJOR key, but the actual key could be the relative minor, or any of the modes, but at least you'll have a starting point to go from.