First disconnect the wire(s) at the horn. Use a 3' jumper , hook to the battery + and touch the other end to each horn. You will know immediately if the are working. If this is o.k. go to step 2.
You could use a test light to test for proper v at the horns. disconnect the wire to the horn(s),connect the test light to the wire and press the horn button. If there is a good light, you have a horn problem.
If you do not have a good light, sawp relays first then go to the wiring diagram to test for problem(s).