[eluser]TheFuzzy0ne[/eluser]
Short of asking for them to fax you proof, I don't think it's possible.
I think you need to work on a referral scheme. That way it's fair, and doesn't matter whether or not the users are in the same household.
May I ask what you Web site actually sells? If I can understand that, I might be able to offer a more suitable solution, as I'm just guessing here.
EDIT: D'oh! I just noticed your signature. I need to have a think and get back to you.
