There are a few easy ways and some other more subtle ways to identify bootleg jerseys. The price, in most cases, is the most common giveaway.  Other things like colors, type of mesh, and design details can be used to identify counterfeits also. I have a lot of knowledge to write and I am updating this a lot. PLEASE email me personally with any specific questions, especially on specific auctions. I can tell you if it's fake or real. I know so much about NFL jerseys that I can write and individual guide for each team. So there's so much I can't fit here.
   My specialty is NFL jerseys. I am an expert on current authentics, Mitchell & Ness jerseys, and also vintage authentic jerseys. I am an artist and I have had an obsession with the uniforms since I was drawing my favorite players as a kid up to today.
I. First, know your team's uniform. Look at photos of the player that show detail of the jersey. Compare it with the item up for bid. It should look exactly like the players jersey.
   A. There are some subtle exceptions. Some teams and players have small details that are different. For example, Bret Favre wears his jersey with no spandex cuff on his sleeves. However, the Packers make all their authentic jerseys sold to fans with the spandex cuff. All teams currently make their authentic jerseys with a spandex or ribbed nylon cuff, except for Colts, Chiefs, Patriots (though the silver jerseys have a navy spandex cuff), Bears, Titans,  and 49ers.
   B. Look at the pattern of the seems, and the pattern of the mesh. Each NFL teams uses a specific type of mesh and seem pattern on their authentic jerseys. Many teams use a diamond mesh pattern (though it looks the same it is heavier) like the replica jerseys use. Atlanta, Carolina, Indianapolis, Miami, New Orleans, Seattle, Pittsburgh, New York Jets, Houston, Detroit, Jacksonville, and San Francisco(only the alternate) use this pattern. Other teams such as Denver, Green Bay, New York Giants, Chicago, New England, Dallas, and so on use a plain-looking mesh with ordinary looking holes in it. And some teams, like Arizona, San Diego, and Minnesota, use have their own unique pattern, they kinda look like Green Bay and New England's pattern though.
   C. Most teams with a traditional style design, such as the Packers, Saints, and Eagles, have a seem pattern between the mesh body and the nylon shoulders that is square and goes straight horizontally, and the mesh and nylon meet where the v-neck collar ends. For some teams, the collar is positioned dipping into the mesh body. However, some teams have this type of collar position on the field, but the authentics sold to fans have a collar that meets just with the seem (sometimes it's different with big sizes, the collar is smaller). Also, some teams use ribbed nylon for the collar when they are spandex on the field. The collar and NFL Equipment logo should never end above the seem, creating a vertical seem from the bottom of the collar to the mesh seem. Below I have a list of the teams who make their authentics a little different from the player's ones.
  1. Carolina, Detroit, Kansas City, New Orleans(except the alternate and the gold alternate), Tampa Bay  - uses spandex collars that the v-neck goes into the mesh body but the store-bought authentics have a ribbed nylon collar that meets at the nylon-mesh seem.
  
   So many teams have subtle differences to the players' jerseys. There's so much more I can say and I my update later. The next thing to watch out for is that on current player jerseys, some teams have decorations that are not sewn on. Many counterfeits have everything poorly sewn, even if it is supposed to be screened. Just because everything is sewn doesn't mean it's real, sometimes, it means it's fake. I will list here the teams that currently have vinyl screened on details.
Buffalo - the sleeve stripes on the alternate jersey are screen printed
Carolina - the panther logo on the sleeves is screened on.
Chicago - the sleeve strips are screened on
Cincinnati - the bottom 3 tiger strips on each sleeve are screened on
Cleveland - sleeve stripes
Dallas - sleeve stripes, though the star in the middle on the blue jersey is sewn
Detroit - sleeve stripes
Green Bay - sleeve stripes
Kansas City - the sleeve strips are screened on since they are normally sold with a free sleeve and not a cuffed sleeve. On the cuffed sleeve, it should be ribbed nylon cuffs which make the sleeve stripes.
New York Giants - red sleeve stripes on the white jerseys
Pittsburgh - the entire sleeve on the Steelers' jersey's is a nylon knit which the sleeve stripes are made by the knit changing threads. Some counterfeits have managed to make sleeve strips that are knit but the entire sleeve is not knit nylon, only the stripes. The whole sleeve is a nylon knit with a spandex cuff. The Redskins' maroon alternate is also made this way.
San Francisco - sleeve stripes and the logo in the middle of them are screen print
Washington - though some of the players have jerseys in which the sleeve stripes are in the ribbed nylon cuff, the authentics sold to fans are made with wider, screened printed stripes with a burgundy or white spandex cuff.
For every other team, everything is supposed to be sewn. There are still ways to tell the bootlegs with teams that have all details sewn on.
 
   There is so much I know spotting counterfeit jerseys that I will start by offering my service of distinguishing counterfeits from real jerseys. When you see a jersey you have thoughts of bidding on, email me with the item number. I will look at the auction and tell you if it is a counterfeit. I will also explain how I know it is a counterfeit. I am prepared to prove my knowledge. If auction you are looking at is a fake, I will also look for a real jersey for you and send you the item number(s) (if I find any). Ask me ANYTHING about NFL jerseys, including vintage jerseys. I have an artist's obsession with the uniforms and what they are supposed to look like since I have been drawing football players since I was very little. NFL is my specialty. I know NBA jerseys as well, though I am not as knowledgeable on basketball but I will come up with an answer for you on weather or not a particular auction is real. I know college football as well, almost as good as NFL. I do not recommend that you ask me about any baseball auctions as I'm not that good with baseball. I have no bias, I just have a lot of knowledge about this stuff and want to help out because these counterfeits are so lame. So ask me, challenge me, don't bid or buy without seeing me first.