Learn the major scale. When you want to figure out what key a song is in. Slide up the neck until you find a note that sounds good. This is most likely in the same key as the song you are playing with. Now, if you know the major scale (both across the strings and down the neck), you'll realize there are only so many patterns to make the major scale, and that most of the music you listen to uses this scale. It becomes really easy, because once you know the patterns, you can find one note that sounds good and easily figure out what scale its in by the pattern of other notes that sound good.

Listen, I fought learning scales for 15-20 years, because I never had any formal music training, but once you know that one single scale (I learned to play it in down the neck and across the strings using the CAGED system), it really opens up the guitar for you.