The only way to do this is to start at the battery and work out. With a volt meter check that you actually have 12.6 volts or more, second put the negative lead of the tester on the positive post of the battery (the post not the cable clamp) and the positive lead to the hot post of the starter and see if there is any voltage showing when the starter is activated, there should be less than 1/2 volt. Next put the positive lead on the negative post of the battery and the negative lead on the starter ground point and again turn the starter over, since there is no relay on this side the voltage reading should be no more than 1/10 volt. Doing this will test for a poor connection in either of the wires or the relay. If everything checks out there then you need to get the starter tested or replaced.