Picked one of these up a few months ago and it's been invaluable to me, so I thought I would share it with the world.
Amazon.com: DROK 2-wire Car 0.56" DC 3-30V 12/24V Green LED Small Digital Voltmeter Panel Volt Meter Voltage Gauge: Home Improvement
Just touch the 2 leads to the pos/neg of your battery and it'll tell you the voltage, I clipped my wires a little as they came really long.
Amazon.com: DROK 2-wire Car 0.56" DC 3-30V 12/24V Green LED Small Digital Voltmeter Panel Volt Meter Voltage Gauge: Home Improvement
Just touch the 2 leads to the pos/neg of your battery and it'll tell you the voltage, I clipped my wires a little as they came really long.