According to a carbed Wiring diagram (pretty universal) the horn button is Blue/Green and goes to the horn relay. The White wire at the relay is connected to a fused source of 12VDC.
The White/Green wire goes to the horn itself and the Black wire on the horn goes to chassis ground to complete the circuit.
You can check the continuity of the horn button segment by connecting a Volt-Ohm Meter (VOM) from the Blue/Green wire at the relay and ground, setting the meter to Ohms (and "beep" if the meter has this feature) and then pressing the horn button. you should read 0 ohms (or hear a beep) if the horn is working. You can pop the center of the steering wheel to visually check the condition of the horn button assembly.
To test for votage to the relay, set the meter to DC, and measure from the White wire at the horn relay to ground for 12 Volts DC.
You can find the wiring diagram mentioned above through Ack's FAQ (link is in my signature) by doing a search for diagram and selecting the 2nd item listed, Samurai USA-spec wiring diagram.