Warning signs

Agency or dating site
Her Agency or dating site with her profile includes other scammers and has a bad reputation according to the reviews of other users.
Her profile
She contacts you first on a dating service.
Her profile states that a man's age, race or income doesn't matter to her.
There is a 20+ year age-difference. Chances of being scammed only increase with the age difference.
She always asks you to contact her via her private e-mail address.
She changes her e-mail address during correspondence.
Scammer talks a lot about honesty.
Home country
Her profile says she is from the USA, UK or any other country, while in her letter she says she is from Russia, Nigeria etc.
Her photos
She is too beautiful to be true.
Most scammers use photos of professional models/celebrities or photos of beautiful women they have stolen from the Net.
You receive a photo of a potential date and the photo doesn't match up (e.g. the photo looks like it is from a magazine or the description the person has given of themselves doesn't match the photo).
She sends photos with sexual content.
Spam letter
She contacts you out of the blue on your private e-mail address and you have no idea where she got your e-mail address from.
Her letters
Her letters do not sound like replies to yours, but instead seem generic.
The letter is poorly written, vague or repeats itself.
She sends pre-written letters - the same that known scammers have written before.
Your name
She addresses you by the wrong name (or the letter is not written to you at all). Your name appears only once at the beginning of the letter.
She avoids questions
She never answers your questions.
She avoids direct questions about her personal life, where she lives, home address, phone number, place of employment.
She falls "in love" with you after only a few letters/emails
Even if you are a great guy, it's too fast. Until you've met in person and spent time together, any claims of love should be treated as extremely suspect. Even if she seems genuine about it, you should seriously consider moving on to a more rational, level-headed person who wants to take the time to get to know someone really well before making any type of commitment.
Money problems
She is having money problems (lost her salary, money stolen, to pay the rent or repairs, to pay hotel bill, to buy a return ticket home from another city/country, is arrested for drunken driving etc.) and needs your urgent help. Often a scammer wants to borrow money to solve these problems, promising to return it to you soon. But this is just one variant of the scam.
She/her mother/grandmother/son/daughter is ill/recovering after an accident and/or needs an operation
You might see her asking for money to help her sick mother or child and she can turn only to you for help as she knows you love each other and are meant to be together soon. It's not appropriate at all in any culture to ask a stranger you've never met to help you.
She wants to study English (or your native language)
Unless you've met in person and things are serious, you have no reason to be paying for her to study foreign languages.
Scammer claims that her English is not so good and needs you to pay for translation services.
Correspondence expenses
She wishes to correspond with you but is poor and needs you to send her money to pay for email services.
She ask you to pay for her Internet access
She might ask for money to pay for the Internet access she uses in an Internet-cafe.
She offers to come visit you
She wants to come visit you but needs money for a visa, airline ticket, insurance, travel-pass, medical reports etc.
She needs money to secure your lodging for a visit
For your meeting with her in her city she might want to arrange everything, asking you to send money so she can have lodging and transportation taken care of before you arrive.
She needs money for "this or that"
Scammers can be quite creative. She needs money for a webcam to communicate with you in video chat, or a camera to take new photos for you, maybe a laptop so she can write to you at any time etc.
Fake verification site
Scammer asks you to visit some site to verify that you are a real person. At this site, you will be asked to provide personal information - address, phone number, documents numbers, bank account details etc.
Money transfer, gold, diamonds, money orders
Father died, left her money but she needs a secure account to put it in and offers 50% to use yours.
She has gold or diamonds but you have to pay all the taxes and fees to bring it to your country.
She may tell you that her employer pays her with money orders, and she can't cash them. She will send you the money orders and ask you to deposit them into your bank account and then wire the money to her via Western Union.
Lives in one country but at the moment is in another
Scammer claims to be from the USA, for example, but is in Africa on business, working for UNICEF or any other organization, visiting parents/friends/relatives etc.
If you do not send money
Letters become more desperate, persistent or direct if you do not send money straightaway. You don't love her and were only playing her and now she is heartbroken and can't trust men again because you are all the same.