As a buddy, I must tell you that's a totally absurd way of doing business. How are you still in business? Logos can come from any possible source, and just because someone creates a logo, it doesn't mean they retain rights to it.
It is good to ask of course, but when Adidas called me to make them up some banners for an event that weekend, did I ask them if they owned their logo and artwork they sent me? Did I ask to see paperwork proof that they owned it?
When a plumbing company that's been around for 14 years, came in to get a small fleet of vehicles done and e-mailed me over the .eps and .pdf files for it, did I ask to see proof of payment for their logo or did I just letter their vans and accept the check?
Just my $0.02 on the subject, you cannot question every customer coming in where all their logo came from. If you get a bad gut feeling, go with it and look into it a bit to save yourself a headache. Otherwise, do your paperwork, do their job, take the check, and move on to the next customer.
And since I'm drifting wayyyy off topic. Might as well throw in that I really like your work Ursta. You like em wild and crazy and I dig it