Tag's are faked as much as Rolex's unfortunately, but fortunately not as well from the ones I have seen. A good way to tell is just the feel test. If should not feel cheap and light.
Also, the model number should be on the back. Google that and see if it matches your watch.
The dots on the face should glow brightly after just a little light. Check to see if it looks like a cheap glow in the dark sticker.
Check the crown for the logo as well.
If you are really interested I can show side by side pictures of a genuine Tag next to a knock off.
and if I remember correctly that particular model has a one way bezel that clicks pretty crisply, if it goes both ways, it is likely a fail.