You can Inspect the 12v terminal on the starter, crimp the connector a little to tighten it up, and use a volt meter. Put the positive side on the starter 12v, and ground the other. Take a reading while cranking. It should not be below 10.5 volts, and 11-11.5 is best. If its low at the starter check the same way at the battery. Then check the output side of the relay. The trouble is in 1 of the 3 parts, and it will only take a few minutes to find.