"A little bit of porn does no-one any harm." "a little bit of porn" is emphatically not the issue. The danger with the fake Hallmark cards (they've been in widespread circulation for years) has nothing to do with porn, it's that they contain MALWARE. Never mind your kids, it's your bank account you should be watching, or any other online services you use, or the sudden increase in your broadband traffic because your computer has been hijacked to send out thousands of spam messages at the bidding of some botmaster in Ukraine...
I've had many real and fake e-cards, and the real ones invariably name the person they came from. The fake ones say they're from "your family member" or "your friend" or some such rubbish. That may not be a hard and fast criterion forever, but it's the first thing to check. After that, the fact they they link to a site that isn't hallmark.com shouldn't be too hard to find out, and then it's time to hit the delete key and move on.