You would want to test for battery voltage on the red/white wire and the blue wire at your starter relay with key on. If voltage is good on both the relay coil is good.
You could then ground the blue wire and if relay cranks the starter the relay is fine.
If the starter relay checks out good you might have to do some checks on the starting circuit cutoff relay, the blue wire goes over there before going to the starter button, which grounds the circuit to operate the starter relay. Your various interlocks are wired around the cutoff relay to determine whether using starter is permitted.