There may be more scammers on the free sites but then again, I think more newly single people are on the free sites, and the good ones probably get snatched up before they get to the point of paying for a site.
I'm a sucker for a well written profile, with good vocabulary, spelling and punctuation AND a good sense of humor. They don't have to write a novel but what they write should be entertaining or interesting. No pictures with their ex obviously cut out. No selfies in the bathroom mirror. And NO to the one guy who was wearing a bathrobe and holding a duck (a live one!), I still don't know what that was about hahahaha.
When in doubt, ask a good girlfriend to read them with you - she may see red flags that you do not.