You need a volt ohmmeter, also called multimeter. Theyre not expensive and simple to use for what you want to do. You just buy or borrow one and set it for "20 volts DC" or whatever is close. Install the probes red goes to red etc.
http://da.harborfreight.com/cpisearch/web/search.do
Check the reading at the battery with the engine off. it should be about 12.8 volts, or anywhere in the range of 12.6 - 13 volts. Now start the engine. Let it idle for a couple minutes and shut off all the accesories like the a/c, radio, etc.
With engine idling, now measure at the battery again. 14.0 volts is what you want. 13.8 is permissible but low. Anywhere in the range of 13.9 to 15 volts is acceptable. Less than 13.8v with engine idling means, alternator belt is slipping, or alternator is bad, so change the alternator.