Use a volt-meter (or a test light as a last resort). Check voltage between the battery posts. Move to the battery cable ends at the battery and check voltage. Same is good, drop indicates a problem. Move the negative lead to the block end of the ground cable and test. If good, move the negative lead back to the battery and start down the positive path. Other end of the positive cable, starter relay, firewall positive connections, etc. Jumping around checking random places is a waste of time, and you should be able to locate the problem in less than 5 minutes if you follow the path of the power and see where it stops.