A lot of fakes are blatantly obvious, like a watch that is supposed to be an automatic and the second hand ticks in one-second increments because it is a quartz. I see a lot of fake Rolexes like this, and they are extremely cheap and awful.
When someone deliberately misrepresents a watch as genuine, knowing it to be a fake, well, that is obviously FRAUD, and this has happened to friends of mine and customers recently. Money lost has been in the tens of thousands.
Without calling them out by name, there is a popular auction site that advises against doing side deals, that is, transactions outside the site because the site warranty and protections do not apply. This is good advice.
As I have mentioned elsewhere, there are certain brands of watches that are harder to detect as fakes. Cartier, Breitling, and others use ETA and Sellita movements, (as well as their in-house ones) with Cartier or Breitling branding on them. If you take an ETA movement and put a Cartier engraved rotor on it, it will look a LOT like the genuine article. Fortunately, a good watchmaker can spot these, as the fit and finish (among other things) will give it away.
These are usually COSC certified chronometers, and that means they are the Chronometer grade ETA movements, which few fakers will go to the trouble of using. That said, I have seen ones that did go to that trouble, and the fake was virtually indistinguishable from the genuine article. What finally gave it away was the thread pitch on a certain part of the case was incorrect.
Thread pitch, that was it.
This lead to checking the serial number, and although it was a valid number, it did not jive with the watch's purchase date nor country of purchase, SO>
Another big giveaway is the serial or model number. Most fakers will not bother to use a genuine serial number and that is the giveaway, but beware that there are some who DO.
Metallurgical composition is another tell. However, stainless steel would rarely be tested for its composition, but gold often is. Something that is gold plated, obviously will wear and look bad, and is suspect especially where it is supposed to be solid gold.
There are also watches that rarely use COSC chronometer movements, and these are the easiest to fake. An ETA movement in the lower grades is easy to engrave with the proper markings, but there are giveaways here as well.
Engravings and laser etched markings can be faked, even on sapphire crystals, so do beware.
I don't want to get into too much detail here, it would be too useful to someone looking to make a fake.
The short version is, buy your seller, but sometimes even that fails. When it does, there are sometimes site warranties that protect the buyer, like you-know-who.
The Authorized service centres will of course tell you if your watch is a fake, but sometimes this only comes up when the watch is getting serviced, and some years may have passed since the purchase date. A certified watchmaker can identify a fake, but recently where this broke down was, a fake was sold as a new watch, and to open the case would void the warranty, so we did not do it. Turns out that the watch was indeed a very good fake, and the movement was a dead giveaway.
Generally, Authorized Dealers are the safest bet, but certainly not the cheapest. Nothing is 100%, as I have seen even dealers and other high end jewellers get caught with counterfeit watches. I saw one recently with a very well known jeweller's name on the warranty card, but the box and papers and card were all fake, fake, fake.
If it looks like a too-good-t0-be-true deal, it probably is, as the saying goes.
Let's be careful out there.