How do you know if its the alternator or the battery?

If jump starting gets your engine started and running, but the car will not start again once you turn it off, the battery is probably the problem. In this case, the alternator is doing its job of keeping the battery going once it has been jumped, but the battery can't hold a charge when the alternator is shuts off.

 When the engine starter does not turn the engine on or the lights are dim, that means you have low battery. Difficult starting the engine because of slow cranking is also an indication of a flat battery.

If the battery is low, it needs to be recharged to restore full power and to prevent possible damage to the battery. The cell plates become sulfated if the battery is low or the battery is left in a discharged condition for several days. As sulfate builds up on plates, it reduces battery performance.

The average car battery life can be anywhere from three to five years, but the exact time any given battery needs to be changed depends on a number of factors.