Before buying the jersey, turn it inside out and examine thoroughly. Observe the seams and hems; the texture of the fabric; the border between the different colors and the printing on the jersey. You are looking for poor needle-work, haphazard sewing basically any sign of poor quality mass production.
Chances are, you have a Smartphone with you. So, go to the online store of the Football club or a reputable store that sells the real stuff. Research on the fabric used to make it and what the real jersey should look like. If there are any discrepancies with what you see online and what is before, there is a possibility that it is fake.
The number printed on the jersey should be part of the fabric itself and not appear as a sticker or cheap printing. Look for symmetry. If it is obviously off balance, it is a fake; if you scratch the printing and it begins to peel off then and there, it is a fake; if you rub areas of the jersey together and the colors bleed into each her, it is a fake
Find one of the a smaller size jersey and wear it in the store;
This is to test the strength of the stitches. Try pulling the fabric lightly. If it tears with minimum effort and the stitches start to run, it is most probably a fake jersey.
So in conclusion, after wearing the jersey, check whether the fabric goes back to its original form, color or does it now have the original texture when you purchased the it? If not then it is fake.