Each designer has distinctive elements that make their watches easy to identify, at least in terms of whether they’re outright fakes or not. Some watches have sweep second hands and some have tick second hands, for instance. Make sure you know the particular model you’re looking at before you use this as a criterion for determining authenticity, however, as the same manufacturer likely uses both movements across different models.
Check the markings on the watch. There should be no errors and the text and engravings should be razor-sharp in terms of clarity. Know the logo of the companies you’re interested in so you can spot fakes.
Buy from a reputable dealer. This is the best way to ensure that your money is well spent.