The true test for something like this is to communicate with the person. Those who becomes defensive at the mere mention that what they show is junk, well that is probably someone who is looking to rip someone off.
Sometimes, you get people who genuinely do not realize that what they have is junk and appreciate the assistance and knowledge you may provide them.
I get a few inquiries a year at work from people who heard that I collect cards. Just about every one has been a "hoard" of late 80s/early 90s junk! One guy brought in a small album loaded with rare post cards from the early 60s. I photocopied the pages and looked them up for the guy and he had several $500+ SP cards that I would have never guessed, solely because they were SPs. He probably had $2500 BV in that little album, all in pretty good shape from when he was a kid!
You just never know. However, there is still really no excuse for not doing some BASIC HOMEWORK either first. Book stores, shops, shows, etc. Then that little thing we call the web. The internet is a wonderful place, packed with good information. If you can't figure out that you either have something of value or a complete bust these days, maybe you are a moron.