how know if my starter is bad

First check the battery at rest the battery should read 12.6 volts otherwise it needs to be charged and tested. There is a test called a voltage drop test that can be done on the large power wire to the starter, the smaller wire brigs power from the ignition switch, it must have battery voltage when the ignition key is turned to the starter position. Disconnect the battery during starter replacement it is not difficult to replace the starter.