http://www.paypal.com/ is good, everything else is bad. Adjust per region and you're set. I don't get the confusion. There's no reason to say paypal.co.uk is also good. American users should use the American version. End of story. Look for the exact phrase
http://www.paypal.com/ on the left side of the URL bar. If you don't see it, run.
Or rather, I get the confusion, but I think the solution is to simplify the explanation. The set of good paypal URL's contains exactly 1 item. Teach someone to look for that item.