1. Low friend count (usually under 50)
2. Lives in another part of the world, but seems to have most of their friends from your area.
3. Their job description does not match their friends list
4. Has a lack of posts (usually because it's only a day old account).
5. The name is little strange and doesn't match their profile pic
6. Their profile pic of themselves is often of another person.
Few know that you can do a google image search by uploading the pic of their profile. I do this by first saving their pic and then going to google. Click "images" in the upper right hand corner. This brings you to another google search box. In the box in the centre click on the camera icon. Then click "upload an image" and then wait for results.
An example today for me was someone calling themselves Tom Jones who lives in Miami Florida (Originally from New York New York). He studied business administration at Harvard. 11 of his 20 friends are from Windsor, Ontario. When I put his pic into google search I get Chris Patton stunt artist.
Friend request denied and FB account reported.