Can't you simply ask to see their ID and make sure the names match?When someone I don't know is using a discount card, I nicely ask lots of questions..Like, are you new here? I can usually tell by the first two numbers if they are new to our store. Or where do you work? Ask them about their store and all that. If their answers don't seem to make sense and they are buying something large, I bring it to AP's attention.