As Rick said you will need a multi meter, set it to V- (volts DC) and you may have a switch nominating the volts eg; 3, 6, 9,12 etc Volts must be set to higher than the battery being tested. OR you can buy a battery tester from Jaycar or Dick Smith, very cheap and easy to use for all types of batteries. My 7 year old enjoyes his job of being the battery tester.
Cheers Daniel.