if it's the battery the car will prolly start hard after a long night or it might even bee dead in the morning. but onve you get it started the car will run off the alternator.
if it's the alternator if you're drivin short ditances you will prolly be able to make it places and think just your battery is low won't notice it that much, but if your takin a long ride , well you're gonna find youself stranded soon the alternator will not be able to charge the battery and the car will just die . that's the hard way to find out
but I believe if you've got the car running and you disconnect the battery and the car just dies. it's the alternator. if you do the dame thing and the car kinda runs on a bit after the battery has been diconnected, it's the battery. don't quote me on this I could be wrong
actually, its my winter beater (dont drive my civic in winter), it doesnt start, but when I jump it, it is fine, but once you turn off the engine and try start it again, it wont start and it needs to be boosted.
just take the car down to autozone/misc. street corner parts store. they have a machine that tests the system and tells you what the problem is. if you have a voltmeter and want to do it yourself, then you can go outside with it and test the voltage at the battery. should be 12-14 volts. then turn the car on and test the voltage at the battery positive terminal and a chassis ground. should be 14-18 volts. have someone (or you can) turn on the lights, the radio, and the AC and test it again. should drop a little bit. if the voltage is less than 12-14 with the car off it is your battery. if it does not pick up a couple of volts when you turn the car on, it is your alternator. if it drops out a lot when you turn on accessories, you may have a short or a bad ground somewhere.