How do you know if you need to change a starter or just a battery?

very simple. if your starter is bad your car battery should be fine.. so turn on the headlights and the car radio and open the Windows. do the lights work? does the radio play? do the power Windows work? if yes then the battery sounds pretty good! if your battery is low on amperage it will not be able to start your car but may work on some eleectronics. if the car will not start but seems to have a charge in the battery. you may want to get a bar or a long metal pipe and hit the starter a bit, this may move the starter enough to get it spinning again (happens alot with cadillacs)