Voltage meter. Either pull it and check it periodically. Twist on a 510 volt meter and fire it. Or learn how it 'hits' over time.
I just ordered a bunch of batteries from day one. Dated them, and put them in rotation. Before I go to bed, and when I get home from work I swap them. They never get low, and they all get used evenly. If I was going to go on a charger free trip somewhere I would bring my 510 volt meter so I could check them easily. And a spare or two in a plastic - secure - case.