I don't have the wiring diagram for your particular truck, but I can tell you that relays are electromagnetic switches. You have one circuit (low voltage) that switches another circuit (sometimes much higher voltage) on and off. In the case of the horn, the button is probably on the low voltage "switching" circuit, and the horn is probably on a higher voltage circuit.
I'm sure others will chime in, but I would think you should be able to test the horn directly without the relay. I'd think all you'd have to do is disconnect a couple of wires at the relay and touch them together. Doing this would simulate the relay completing the horn circuit in response to pushing the horn button. Definitely look at the wiring diagram for your truck before messing around with anything!
If interested,
here is a link that explains how relays work.
