Subaru pit jersey
 

How can you tell the difference between a real and knock off?

the tag

describe the real tag.

Authenticity-
 

Look for the tag on the side lower seam of the shirt to say "SWRT" or "Prodrive" and the tag on the inside under the collar as well. Just like others suggested the tags should show the authenticity or not. Either way as long as it looks good a team shirt would still be awesome! ;)