Here is how I keep tabs on my house battery voltage. Note the yellow volt meter mounted next to the electrical outlets over our bathroom sink. It is a $15 (maybe $19 now) Radio Shack digital volt meter. If left on it shuts itself off after about 1 minute of none use.
Below is another digital voltmeter I found a few months ago at Micro Station. It only costs $5. I don't think it will shut itself off though. Small, compact. I bought five of them and keep them in all my tool boxes. No, it is not heavy duty or as good (small, light, light probe wires, etc) as my "real" volt meter but a bird in the hand is much better than a bird in the bush when you have a problem and need a quick AC or DC voltage check.
PS: this is the first time I've posted an image so hold your breath