I just use my ears. The degradation from a low battery is hard to miss. This being said, I once changed the battery while at my tech's for other work and for kicks, he checked the voltage at the time. It was just above 6 and the buffer was working still. This was, though, a CAE buffer, not one of Mike's (I've since swapped out the CAE for a Wald).
"For me, I think the only danger is being too much in love with guitar playing. The music is the most important thing, and the guitar is only the instrument." Jerry Garcia