test your battery. test it car off, should be 12 volts or higher. then test the battery when its running it should be 13 volts to 14 volts, give it a little gas to see if the volts go up if they are still at 12 or not much higher then when running. low volts running is a bad alt.
some parts stores also have a load tester to test the battery, they can also test your alternator if you take it off and bring it in. same as the battery. you need to bring it inside the store.
and as far as rebuilding your old alt, they are usually cheaper. ive gone that route in the past as well. its nice when they have one ready to go. but have gone back in as little as a hour to pick them up, and they look like new too.
