The "Paris" tag is sewn into the collar of coats and jackets and suits. It is a silver loop with "Paris" embroidered on both sides. There is a thin black mesh loop that covers the silver loop.
Example (click to enlarge):
The tag should be about 1.5 inches long (4cm). In the example of the fake tag above we have several things wrong (other than the creepy thumbnail). This fake tag is a stubby little thing. The letters are also further apart and larger in scale than the authentic tag. The other thing that is off is that they are 'upside-down.' When the jacket is on a hanger and the tag is pointing down the "P" should be the closest to the collar seam. If you pull the tag up (so it is pointing towards the top of the hanger) and read the backside the "S" should be the closest to the collar seam. The fake tags often have this reversed, so it is a sure signal. However, I have heard from several people who have bought Dior jackets (that they swear are authentic from authorized retailers), that occasionally an authentic jacket will have a reversed Paris label. If this is true it seems to be a rare occurrence. Keep this in mind and use your best judgment.
Example (click to enlarge):
This is where that little piece of knowledge comes in handy. The fake on the left is a pretty good copy. If you click the image and enlarge it you can see the letters are a little shaky, but all-in-all it's fairly convincing. However, the tag is reversed. The tag is pointing down and the "P" is not the closest to the collar seam. Granted there may be exceptions to this rule for authentic jackets, but it is a good red flag because so many fakes do have it reversed. Upon closer inspection of the fake above you can see that the letters are too close together, the tag is too wide, and the writing is not centered on the tag.
Example (click to enlarge):
This example above is of another fairly accurate fake, but again, the fake letters are reversed and they are a little bolder and also closer together than the authentic one. The authentic Paris tag has a little more spacing between the letters than this fake example, but not as much spacing as the fake example below.
Example (click to enlarge):
This is a decent fake. The letters are going the right way, though it's not a very clear picture. Even so, you can see that the letters are spaced too far apart. It should appear as "PARIS" not "P A R I S"
Example (click to enlarge):
The fake above is kinda pathetic... I almost feel sorry for the little guy. However it does have one thing correct, the letters are going in the right direction. The tag is pointing up towards the collar and the "S" is the closest letter to the collar seam (if it were pointing down it should be reversed). This knock-off is also missing it's stylish mesh loop.