How do you tell what key?

do you know your major scales? if not, learn your major scales (read them as notes, not tab) and then you'll know your keys. also, you can learn your circle of fifths, clockwise it goes C no sharps/flats, G - 1 sharp, D - 2 sharps, A - 3 sharps, E - 4 sharps, B - 5 sharps, F# - 6 sharps. Counterclockwise it is the circle of fourths, C no sharps/flats, F - 1 flat, Bb - 2 flats, Eb - 3 flats, Ab - 4 flats, Db - 5 flats.