The price is a good place to start. If you see that the price of the product seems a little too cheap, then it's probably a knock off of the real thing.
You just spoke my mind. The thing is, if the price is exaggerated, then there is a high chance that it is counterfeit. If people spent more time, building their own brand, other than modifying another company's name, then we'd probably be in a much better position.