Sometimes the price is too nice. A Tag Heuer retails for approximately $800 or $900. If you're buying the watch for $100 or less, don't expect it to be real.
Check out the real deal. Visit a store and examine a Tag Heuer watch that is guaranteed to be authentic. Feel the weight of it; a fake will feel much lighter in your hand than the genuine time piece.
Light bright. Tag Heuer watches have a special material (luminova) that makes for a bright face at night. Fake watches will have a dim light compared to the real thing or not have a night light at all.
Find the weakest link(s). Tag Heuer watches have links made from two distinct pieces, a left and a right piece joined in the middle. A lot of fakes have an indentation to make the links look like they're two-pieced, but in reality are one.
Look for pins. The real watch won't have any visible pins holding the links together, but a fake most likely will.
Look at the fine print. Use a magnifying glass (or a jeweler's loupe) to view the face in extreme close up. The printing on the dial should be crisp and sharp. Fakes will have letters that bleed or appear fuzzy.
In general, stay away from flea markets, unknown online retailers and street vendors. It's highly unlikely that they are selling the real thing. Visit Tag Heuer's website to see a list of approved retailers.