Find out if someone is a scammer
Well it's hard to find out, because scammers job is to mask itself as honest person. And their actions are well thought and studied in advanced. To catch one in scamming you have to be better at the game than they are, so that you know what does it mean what is they offering to you (example: if someone is offering you an Iphone for 100€, when lowest price out there is 300, chances are, they aren't buying one for 300 and then selling it to you for 100).
Or you need to know something about the nature of business (business practice), like if someone is selling new Oakley glasses, when in fact Oakley doesn't sell their glasses threw third parties.
As person them self is the hardest. You need to do your home work as it is said. Check personal info, history, background etc. Even big agencies and private detectives doesn't always get it right. That's how hard it can be. Especially if one is professional. For amateurs you can catch, by asking a lot of questions and checking for inconsistencies or contradictions. Also you can choose options that they are not prepared for (I will pay threw pay pall; Ok, give me email and I will send you transfer request; Request, what, how .... request!?!!).
These are just few most common ones. However there are unlimited options in the field and that's what makes it interesting.