When initially setting up web hooks for
http://brewdega.com, I registered a
http://requestb.in/ URL to collect a few web hook POSTs from BreweryDB.com. I then inspected those POSTs and created several Rspec request specs to mimic those web hooks. After that it was a simple matter of driving out my code to make the tests pass.
When I finally enabled the hooks in production, they worked without a hitch!
I'm not sure what stack you're on, but hopefully your test framework of choice has a way to do lower-level HTTP API tests.