In my opinion, the best way is to go to the Tag Heuer web site and look at their catalogue as a comparison. You can also search google using the serial numbers you are given as part of the description - make sure you see them enscribed on the watch rather than trusting what you are told. If you do not find the same watch by searching elsewhere, do not touch it.
Also, make sure you have all the correct documentation with the watch - the case, instructions, warranty card and where possible the original receipt. If like me you are spending a lot of money on your watch you are likely to keep as much information about it as possible; if even only for insurance purposes. However, just becasue it has the box, certifcates and warranty card does not mean it is genuine. These are for regularly for sale on Ebay and can be attached to any fake watch an unscrupulous Ebayer wants to.
There are also watch forums out there http://www.timezone.com/ is really good and has brand forums where you can ask for advice about any watch and get some decent and honest replies.
My advice is to be canny. Authorised dealers are dearer but if you are patient and look around, you can find bargains out there. I saved a lot of money on my Monza - it was half price and well worth the wait and guaranteed genuine.
I love Tag watches so felt compelled to write this having broused around for a while although I think this advice will be the same for any luxury watch you are considering buying - be careful and as usual if its too good to be true then it usually is!