Talk. Phone conversations can often unravel a fake. When you hear this person on the phone, note whether they have a slight accent and use awkward phrases; if their accent does not match their supposed origin, be suspicious. Ask them probing questions and trust your gut instinct about the validity of the replies.
If phoning, beware a cell phone number that does not match the area in which they claim to live. This often means that the person is not in the same country at all. Match the cell phone number and the area code with the state or province they claim to live in.
If you spot a number discrepancy, beware excuses. They may tell you they've just moved or didn't bother to change it when they did because it would be too hard to contact all their friends with a new number.