I'd recommend keeping the name/phone number/email of your "certifying" dive shop handy. I've occasionally had to contact a shop to verify certification, when they've forgotten or lost their card. A week or 2 ago, someone showed up at my door wanting to rent gear, and while I tried to look up his info online (he said he was PADI certified, so I should have been able to find the information), I asked him what I thought was the simplest diving knowledge question I could come up with......"what is a safety stop?" And he said "oh, it's been a long time, I'd have to look it up...isn't it where the police check if you've been drinking?" So I thought perhaps he never actually HAD taken a course. A (legal) way around that is to have them sign up as a "discover scuba diving" participant.....the people who just want to try out diving, and don't intend (at the moment, at least) to take the OW course. Unfortunately, he didn't want a guide, he just wanted to rent gear, but without any way to verify his certification, I couldn't do that.