Unfortunately scammers like the ones you've described are a fact of life on all social/dating websites. We do our best to keep them in check but it's a losing battle. That being said most of the people here are not scammers but that doesn't mean you should trust them. Trust is something that has to be earned. No matter how wonderful a person might seem you should be very cautious dealing with someone new.

I like to think I've gotten very proficient at identifying scam profiles. There are a few red flags I'll look for and while none of these on their own are enough to condemn a person they make me suspicious:
1. Anyone in/from Africa
2. Photos that appear to depict a porn star
3. Wanting to be my slave without ever having talked with me
4. Poor language skills
5. Isn't consistent about key facts about her life
6. Doesn't seem to know basic facts about where she lives
7. Wants to move the conversation off S.F. to Yahoo or some other mail service ASAP
8. Doesn't seem to have read my profile or posts
9. Profiles that consist of lists of fetishes

When talking to someone new, I'll check out her profile and then ask her a bunch of "innocent" questions about her life. For example if we're chatting I'll ask her what time it is to see if her answer fits with where she claims to live. I'll ask about the weather, after checking it for her town online. I'll ask where she went to school or where she works. I'll try to see if any of the answers she gives don't seem plausible based on her what else she says. The next time I talk to her I'll ask some of the questions again to see if she says the same thing.

Of course if she ever asks for money, plane tickets or anything comparable she goes right in my scam pile.