@Carl16,
Just ask for it in your pre-booking conversations with potential guests. I have done this before quite often and don't send pre-approval until I have this information and this is even for small groups [4-6]. Genuine guests are always willing to forward this info, it's very relevant and nothing less than would be required in regular commercial accommodation anyway.
Another box to fill in won't solve this trust issue.
Thorough communication and a good connection between host and guest is a key feature that will reduce these problems.
Sincere Regards, Christine.