If you want an easy way to find them, pick a starting note. Then, count 3 semitones up and you have the next note. Repeat until you get 4 keys (or get back to the same note an octave up).
I some time ago made a list of interval signatures. It helped me greatly in figuring what kind of chord I have before me, and consequently in analysing. there 12 semitones in the scale.
So, a major chord is defined by this signature
4-3-5
meaning 4 semitones between the first and the second note, 3 between the 2nd and the 3rd, 5 between the 3rd and the 1st again (one octave up).
The minor is
3-4-5
and the diminished is
3-3-3-3
I have a larger list, but these are the basics anyway, and anyone can make this kind of calculations.
Alex