How to know altrnater is bad
Hook up a volt meter to you battery, take a reading, it should be around 12.5 to 13.2 volts, if voltage is lower than recharge battery. Start engine and take another reading, at idle the volts should be 13.7 to 14.2 volts. This a good voltage test. To check the amperage output you really need either take the alternator off and take it to someone who can test it's amperage output or take the car to a shop that can do it.
You can purchase a 100 to 150 amp carbon load tester at most larger automotive parts stores. But the voltage check above is quick easy test. On most vehicles there is also extra wires besides the 4 - 10 gauge wire that charges your battery. One will be the "excite" lead. If this wire is faulty it will not allow the altenator to excite and start producing power. I will read 12 volts with the key in the on position. (Some altenators can self excite if you go over a certain rpm)