Get a digital multimeter. An adequate one can be found at
Radio Shack. Set it to the DC V setting, with the range
switch (if any) set to the next number over 16V. Make sure
the black wire is in the COM jack, and the red lead is in the
V/ohm/A spot. Hold the black probe to the - terminal of the
battery, and the red lead to the + terminal. Wiggle the leads
around, pressing them against the terminals, until the reading
stays steady, only changing by small amounts. Read and record
the voltage.