It is indeed difficult to tell whether it is fake or not by just looking at the outsole, as some fakes have started using the prominent Xs, whereas, to an untrained eye, it can be thus difficult to tell whether the pair is real or not. Thus, whenever you purchase, look at the Big X size on the outsole, look for the logo which will be in a really small pattern, and take a look at the overall shape of your Birkenstock.
(Image Credits: Pinterest)
