when testing a horn, i hook up a jumper wire to the horn's spade connector, touch the horn or it's mounting bracket [made sure there is clean material, no paint or rust] to the negative post of the battery, and just "tap" the jumper wire to the positive battery post.
if the horn is good, you will get a "beep", or high or low note sound from the horn.
i do not hold the jumper for a long time on the positive battery post.