For email.radio, I tend to just match the IP location to the address listed on the license. I also thought about using the navigator.geolocation location API to automatically verify users, but that could easily be spoofed.

I do know that some use postcards and make you upload your official license. EchoLink has an option to charge your card $1 to match your billing address to your callsign address automatically.

https://www.echolink.org/validation/validation_ccauth.jsp