It is really a matter of listening and analyzing the chord progression you are playing over. Take a look at my up tempo blues lesson
http://www.guitarmasterclass.net/ls/up-tempo-blues-jam/Here I do this, start with minor, introduce some notes from the major scale as well. It will not always sound good, so you need to experiment and get familiar with it.
Take any of the blues lesson's backing tracks and practice major for a while, then do the same with minor, when you are familiar with the sounds, you will be able to tell what sounds better when, and when to change from one scale to another.