I have digital ESV (expanded scale voltmeter) as in Exeter_acres link, as well as a couple analog that Tower gave away for free (a few years back when the daily specials really were!) They put 250mA load on the battery and give an approximation of relative life - use it wisely. ;-)

I also have a VoltWatch2 on a couple planes now. I used to poo poo them, but they do give a fast visual.