You can test a car battery with a voltmeter pretty easily. This may not give you reliable information about the battery, but it will tell you if there's a problem with the battery or alternator charge to the battery.
Adjust the settings on the voltmeter for a car battery check. There are so many types of voltmeters and settings to accurately describe, but you want it on "voltage" or "volts" and "DC."
Locate the battery in your vehicle. Most car batteries are in the engine compartment.
Place the red lead of the voltmeter onto the positive battery terminal (red wire) and the black lead of the voltmeter onto the negative battery terminal (black wire). The voltmeter should read between 12.5-12.8 volts with the engine off. Amperage is what the battery uses to start the engine, however. So even if the battery is showing the proper voltage, you may still have an internal problem.
Start the vehicle, turn on the lights and radio and blower motor. Repeat the test. The voltage should read between 13.5 and 14.5 volts depending on loads and battery size. Anything lower can indicate a problem, but it will still not point directly to the battery or alternator.
Disengage the ignition or injection to your vehicle for a more accurate test so it will crank, but the engine will not start. Have someone crank the ignition for no more than 15 seconds with the voltmeter attached to the battery. The reading on the voltmeter should remain at 9.6 volts or higher for the duration for the battery to be good. If it is lower, it's time to replace the battery.
The battery tester is a much more accurate test on the battery and the charging system. It can tell the difference between a bad alternator and a bad battery. It can also determine the life expectancy of a battery. In other words, a battery at 50% life capacity may still last a few months of warm weather and you'd be able to get a little more out of it before prematurely replacing it. Also, the battery tester can pinpoint a charging system problem before it renders your car dead on the side of the road. Try the voltmeter test to see if there is a problem, and if there is, have a free battery test performed at most local auto parts stores that sell batteries. These places rarely charge for the service in the hopes they'll be selling you a battery or alternator.