Learning how to understand key signatures is an important skill for a guitar player because it enables him to better understand the music that he plays. Here are the basic rules for determining what keys are represented by each key signature:
1. The major and minor keys represented by two key signatures should be memorized: (1) no sharps or flats: C major and A minor; (2) one flat: F major and D minor.
2. For all the other key signatures, follow these steps to find the key:
If the key signature has one or more sharps, go to the last sharp. Going up to the next note will give you the major key and going down a third from that note will give you the minor key.
If the key signature has more than one flat, go to the next-to-last flat. That flatted note is the major key for that key signature. Going down a third gives you the minor key for that key signature
This PDF illustrates how to follow these rules for all the major and minor keys represented by all 15 possible key signatures.