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).
Disclaimers: I'm happy to share whatever relevant gear knowledge or experience I have. Genuflection/payment/proof of worthiness neither sought nor accepted. I don't design, build, market, sell or endorse for pay any new music product or service. I occasionally sell personal used gear. I am an unpaid endorser of SoundScaper Productions and Godfrey Daniels Guitarworks.