1.The most major give away is the stitching of the sockliner, it is much further apart than the real things, and various other stitching that you can see inside is also not as straight.
2. On the bottom of the shoe where the Nike + is located, where the outsole meets that clear plastic, the rubber seams are not straight lines. They look melted/fused instead of a nice clean attachment that the real ones have. There are these six little marks that look like screwdriver slots, they are at different angles from the real things.
3. The air bubble is a different kind of plastic, and when you look from an angle they are not smooth, you can detect a variance in the thickness throughout it, and it is not as soft. Real Air max feel more like a medical tubing, and have a consistent thickness.
4. Larger shoe sizes are easier to spot fakes in, because the laces used are the same size across all. For example, I wear size 13 and the laces aren't long enough to go through all of the eyelets.They leave the top two eyelets unlaced. You can also see that the flywire is not threaded the same way, and has no elasticity, where real flywires have tension to hug your foot.
5. When actually wearing them, they feel different. In my size 13 I can feel pressure against my the sides of my toes, so they are not true to fit. The dynamic flywire therefore has no function at all. The cushion is not as soft, and I'll tell you right now that I would never use the fakes for running in, I think they'd damage my feet.
Now, all of these things are extremely hard to spot unless they're in your hands, so if you are wearing these just for cosmetic reasons (I got these in a yellow color that Nike never made so that I could specifically match them to this Bruce Lee themed running pants I wanted lol), you'd otherwise never know they were fakes. All of the fabric used in the product is an extremely close match. So if you buy the fakes, they're good for show but don't use them to run in. I would actually go as far to say that some of the assembly is better than my real ones. In my legit Nikes there is a good sized bit of glue that fell on the toes, it looks like a stain or something and the fakes are absolutely flawless in that sense.