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.