I have a couple multimeters but just don't carry them around with me.
It's not a substitute for a multimeter but I have a bunch of pocket voltage indicators that I use at home and bring with me that I periodically use to check my battery level. I can usually tell when my battery goes below 3.7v but I like to get into the habit of checking anyway.
510 Voltage TankoMeter - Avid Vaper