(In no particular order)

Do any of these apply to the person you're talking to?

Is their photo a very obvious modelling shot?
Firstly, let's be realistic here.  How many models need the help of an online dating site to find someone?  Honestly?  If you look that good there's no way you'll need to be looking online for love on some dating site.    Also, if you're like me and have more belly and less hair than you did a few years back, is someone half your age that looks like she models for a living REALLY going to be interested in you?  Really??

Do they claim to be "but currently in West Africa" for a job/charity work/modelling assignment?
This is a common lie told by scammers.  Well, not really a lie.  They ARE in West Africa.  The truth is that they were never anywhere else before to make then "but currently".

 Do they sound as they should?
I'm an educated, white 30 something male from the UK.  I SOUND like an educated, white 30 something male from the UK.  I have a British accent.  I start my sentences with "I am" or "I'm", and not "am".  I would say "I'm an only child" and not "am the only child of my parents".  I'd say "My name is XXXXXX" and not "am XXXXXX by name".  Only people from certain areas of the world speak this way, and they're not white UK or American people.  If they claim to be in the UK or USA and speak the way a West African would then it's a huge warning sign something's wrong.

Do they want to get you off the dating site and into IM/email?
Scammers often have their accounts on dating sites closed pretty quickly once someone reports them.  Because of this, they try to get you off the site as quickly as possible so they still have contact with you when their account gets deleted.  If they give you their email address in their opening letter to you, then it's a pretty good sign  it's a scammer you're dealing with.

If the girl is in Russia/Ukraine, did she approach you first?
Real FSU women don't make the first move.  I have it on authority after speaking to an expert that it's a cultural thing.  They always wait for the man to make the first move.  That's just the way it is there.  Anything else should make you suspicious.

Are the pictures they send thumbnails?
Who sends thumbnails?  People who can only steal them off a modelling site is who.  "Real" people send good photos, not something that looks no bigger than a stamp or like it was cut out of a magazine.  Be suspicious.

Can you Google any parts of their information?
If you Google their email address, name, parts of their letters and it shows up on a blacklist or a warning site, then you're dealing with a scammer.  If you find out yours is a scammer, then post their details up.  Help warn others who may be suspicious. 

The $64,000 question.
Do they ask you for money?  Here are some of the reasons they ask for money off you for.
 internet cafe/internet provider fees 
needing money to  come visit you
having their purse stolen
BTA (Basic Travel Allowance) a made up fee scammers ask for
hospital fees after getting run over/mugged/alien anal probed
college fees
toiletries/tampons (No kidding! One claimed she needed $50 for a month's supply!!)

If any of these are true, then drop them and report them.