Use a multi meter...
Check the volts of the battery itself, if low, theres your problem.
Ground to same location your ground cable goes to for the following testing(on the block)
If you read low right away, then check using a different ground location, if it changes, then you have a bad ground issue.
Check the volts at the battery side of relay. Should match the battery dead on, 12.5.
If it does not, check your cables.
Then check the other side(starter side) of the terminal, should read 0.
Then press start, that should now jump up to 12.5
If it does not, then your relay is bad
If it does read 12.5, carry on with testing.
Now check the volts at the starter, when you press start, this should also read 12.5.
If it does not, then it could be a bad cable from solenoid to starter, or a bad starter.
Hope that helps.