I'm not sure how far you need to check, but the last deal I made, I just went here:
204.254.113.252/ and verified that the place I was sending a rifle was a current FFL holder and the address matched.
I don't think FFL holders will want to give you the whole license number. Too much fraud out there. But I don't think they will mind giving you the first 3 and last 5 digits. I called the dealer I was sending the rifle to. He wasn't sure how to handle it either, so you may need to explain why you want those digits.
Everything went well on a trade I did through the E/E forum :)
Edited to add:
Oh, and you can ship a rifle directly to an FFL dealer. If you ship a handgun, it must go from dealer to dealer for the transfer. The various carriers have info if you want to search for it. I went with FedEx. I think they said the only requirement is that no ammo is included. I played it extra safe since I was sending other stuff and sent the carrier assy separate with a mag and carry handle.