I bought a brand-name watch but it did not come with a warranty, so I think it is a fake and I want to return it.

Just because there is no warranty does not mean the product is a fake. In most cases, with a brand-name watch that requires an extended warranty, a warranty card is given to the customer upon sale of the watch, but there is no law that requires it. If you are concerned, the first thing to do would be to go back to the store where you bought the watch with the purchase receipt, and ask them to give you a warranty card. Also ask them to explain why the watch is genuine.