There are many different ways to tell if a pair of sneakers is a knock off or not.
The safest way to do so is has the shoes checked by people who actually know their stuff.
However, there are a few things you can compare of see that will tell you if the sneakers are fake.
First and for most, the price point of the shoes. Fake shoes are generally made with a very low production cost, mostly because they do not input so much of the money into RnD, and the material was cheap.
With these reason, their profit margin is usually really high, so even if they sell it for prices at below retail, there will earn a lot. This is why generally, fake shoes are cheap, and they usually sell below the market value of the shoes. Therefore, if you saw a pair of sneakers online that is BNIB or DS, and suspiciously cheap, it is better if you avoid it. If you decided that you want to spend your money on them, know your risk.
In general, if you buy an original sneakers that are worth reselling, their price point will be higher.
People in general do not want to lose money. Some may see a pair of shoes as a bad investment, and just want the money back and do not mind taking a little bit of loss, but it is a very rare case. People who want to break even by selling their shoes have to keep in mind the transfer fee, and the cut that the middlemen that they used taxed these sellers, so the price point will be higher that the original price in store. People who are in the shoe game for the money obviously value the time and effort they put into getting those shoes, so whatever price they put on top of the price of the shoes are the valuation of their effort. If you missed a release, and you want to buy them, you may get them at a good deal, but it is almost always never at the original retail value. If you got a pair at a good price and they are the original pair, good for you. If not, well, the price point says it all.
Fake shoes may have packaging boxes that looks very real, but the boxes usually have different tag like style number or serial number from the shoes. They may or may not be damaged, but there will always be a flaw in the box.
One of the most obvious difference between real sneakers, and fake ones are the material. Real ones are made with materials that are more expensive. The shoes will feel more sturdy, and comfortable.
Real sneakers were made in manufacturing companies using cutting edge technologies to make sure that the shoes produced are consistent in quality. They went through quality checks, and therefore, there are very little defects. When compared to fake ones, they looked better in details, as there will be many defects in the fake ones like some element of the shoes may be crooked or not shaped or cut properly.
I am not an expert on shoes, and this is based purely on the experience I had on buying sneakers. Also, this probably is not the first post regarding fake shoes that is created. Im just exercising my right to freedom of speech. :)