Just to start off with the obvious sign are chrome buttons and a chrome screw down crown, genuine tag f1s have black ones

the next sign is the back of the watch is more fuller on a genuine tag than a fake

.A genuine back.

A replica back ,usually smaller and flatter.

Another point on a genuine tag f1 (left)the date will cut into the second markers to produce a larger date window,on a replica (right) the date window is smaller and does not cover the second markers.

A genuine will say 200 meters on face and a replica will say 200m usually,also the stopwatch is not one tenth of a second operational on a replica.a genuine second hand will be at the six o clock position

Another sign of a genuine tag f1 is that it will have tag heuer stamped on the side(except CAC1111) first issue,replicas never do.

This little guide is based around the tag heuer formula one model CAC1111.BA0850.

a google search for fake tag heuer will find websites full of copys that you can compare with,one last thing a dealer warranty card should be stamped not filled in in pen.