1) Ask yourself if the pic looks fake.
2) Look for indications of someone taking a pic of a computer, like the edges of the screen. See above.
3) Look for more indications of someone taking a pic of a computer, like contrast lines. See above.
4) Look at the girl's profile. Is she a hookup whore? A PC Pic-Peddler? Lesbian? Probably a fake.
5) Look at the guy's profile. Can the body pic match the face? There any body AND face pics? Does it look like he took a pic of a magazine? Probably a fake.
6) Ask a friend if they believe its fake. Most times a 2nd opinion helps.
7) If all else fails... mail me the person's screen-name and ask me directly. I have a keen sense of these things and I love catching folks perpetratin'... like "Hotflababe" you see above.