14 CFR 137.19 explains the demonstration you need to complete. Most agricultural operations will have a Chief Pilot who has demonstrated tot he FAA in order to obtain the certificate. He is then responsible for checking you and issuing you a "pilot card" just as we do in Part 133 operations.
I do not know of any schools that teach it these days, and if they did, the sign off is useless as each operator is required to check their own pilots.