Find the relay and test that by swapping out with the other headlight relay. If the relay is OK:
Bridge the two fat wires into the relay plug, see if the high beams come on. That proves it's not the actual headlight connectors or the bulbs.
Switch high beams on, check if that makes 12v come into the switched live. Checks the switch and the live feed.
Check if the switched ground gets through to earth. Self-explanatory