Turn off the car and all accessories
Set the multimeter to DC volts
Connect the red probe to the battery positive terminal
Connect the black probe to the battery negative terminal
Read the voltage on the multimeter
Check for about 12.6 volts or higher for a fully charged battery
Check for about 12.4 volts for a partially charged battery
Check for 12.2 volts or lower for a weak battery
Have someone start the car while watching the multimeter
Check for the voltage to stay above about 9.6 volts during cranking
Start the engine and read the voltage again
Check for about 13.7 to 14.7 volts with the engine running
Remove the probes from the battery terminals
Interpret low readings as a sign the battery may need charging or replacement