The best way to figure out the scam ads is to read lots and lots and lots of ads over time.
Seriously though, when you've read quite a few of these, you eventually get the knack for which ones are legit, and which ones are obvious scams.
One thing you can look out for though, that seems to pop up a lot, are ads that pretty much blatantly say "come to <insert dating site here>, and make a profile. You don't need a credit card". These always set off my "spider sense".
Also, a lot of ads that blatantly say they are young and horny, and then show you a series of very "professionally" touched up photos of very beautiful young ladies, and then give you an email address like "
needsexnow@gmail.com", or "
youngandhot@gmail.com", that sort of thing. Pretty much always one of those phishing scams as mentioned above. You send an email, and bang! (not in the good way

) They send you an almost automated reply asking you to join up on some "hook up" site and eventually get you to go to the "premium" section, where you have to give up your CC details to gain access.
As I think of more, I'll post here.