How do you know from the key signature if the key is major or minor if the signature is the same for each pair?

Usually you look for accidentals, for example if there is no key signature, it's either C major or A minor. If you notice G#s in the piece then it's A minor.

You can also look at the first and (more importantly) last notes or chords of the piece, usually they will be notes that are in the chord of the key signature. If the last chord in the music contains CEG then its probably major, if it contains ACE then it's probably minor.

You can't tell from the key signature. You're going to have too look at the notes/cadences to get a better idea. sometimes you can tell by the accidents, especially if it is melodic minor or something. Check and see what notes you start/end on. Most likely its going to be a tonic chord in whatever key you're in

Hmmm... which keys have the same key signature for both major and minor? I can't think of any.

http://en.wikipedia.org/wiki/Key_signature