1. If it seems too good to be true, it probably is. If you are trying to buy a Michael Ballack World Cup 2006 shirt, and the seller is only asking $15, it's fake. End of story. No seller in their right mind would sell the real thing for that price when it's worth so much more than that. If, however, they're selling the same shirt for $50-$60, there is a remote chance that it could be real and that you're just getting a good deal.
2. NEVER buy from Thailand. People may tell you that they are the real thing because they are coming directly from the Nike/Adidas/Puma/Kappa factory, but that is just not true. They are coming from sweat shops completely unaffiliated with the aforementioned brands. In fact, I don't think any of those brands even operate out of Thailand. Nike, for one, has factories in Portugal, Bulgaria, and Honduras - Not Thailand. If, on ebay, it says "ships from Asia" or the seller is located in Thailand, move right on to the next shirt. One out of every hundred Thai sellers may be real, but that's not a gamble worth taking. Thailand, Singapore, Malaysia - they all spell trouble. The tricky part is that these sellers will often say they are registered in the UK, when in fact their product is coming from Southeast Asia
3. Pay attention to the wording. For instance, look at the Morientes shirt. It says "Brand New Soccer Jersey with tags. High Quality Soccer Jersey. Satisfaction guaranteed." They are not lying. It is all of those things. But what it DOESN'T say is "Authentic Adidas Jersey." It doesn't even say "Replica Adidas Jersey." Since they aren't even making claims that it is authentic, they aren't ripping anyone off by selling a fake jersey. They are, in fact, selling a "brand new soccer jersey with tags." Sellers will also try to trick you sometimes by saying "embroidered Nike Logo." That is also true - they are selling you a shirt with an embroidered Nike logo. But that doesn't mean that Nike is the one who embroidered it. If, on the other hand, a seller says that a shirt is "100% authentic licensed Adidas football shirt," there is reason to believe they are telling the truth. Some sellers even go a step further by saying "I don't sell Far East fakes."
4. Look at the seller's other items. If they are selling brand new, current shirts of the biggest clubs and national teams, with and without names, from every major manufacturer, it's probably a seller of fakes. Sellers are more credible if they sell shirts from several differnet seasons. Also, if a seller is primarily selling one brand, it is a good sign. That means there is a chance that they bought what is called a "job lot" from the manufacturer. If a shirt is about to expire, the manufacturer (say, Umbro, for example), will allow people to buy hundreds of the same shirt to sell as they please at a cut-rate price. These are completely authentic jerseys, but you'll rarely find job lots of the newest shirts. If a seller is selling the most current shirts, but they are at very high prices - maybe $10 less than you'd find in a store - they are also probably the real thing.
5. DO NOT TRUST FEEDBACK SCORES. Feedback stores are a good indicator of whether you will receive your item, but they do little in the way of proving a shirt's authenticity. For one, some people are OK with buying fakes. In that case, they would buy a fake, receive it, and leave positive feedback. Other people may buy a shirt, not knowing it is a fake. In that case, they could buy a fake without realizing it, still be happy with it, and leave positive feedback. If a buyer is unhappy with the shirt they received, they may still be reluctant to leave negative feedback for fear that they'll receive negative feedback from the seller. Generally, most people will only leave negative feedback if they didn't receive their item at all.
6. Use a little bit of common sense. If you really want a Bristol Rovers shirt, you'd have a tough time find a fake version of it. There are hundreds of teams that the fake manufacturers just don't bother making. If you want the real thing, as soon as it comes out, you're probably just going to have to pony up and pay full price. If you can wait a couple years, though, you'll be able to find that shirt you always wanted at discount prices - right before it's out of date.