If the stitching on the letters looks "puffy" rather than flat.
Here's a good example...
See how visible the stitching on the left jersey is? That's a fake. The letters themselves usually don't sit well on the jerseys when they're fake too, for the same reason.
The stitching on the logo on the front of the jersey is also noticeably "puffy".
For the fight strap, on the fakes, it's thinner and sewn directly onto the jersey. On the authentics, the fight strap is sewn onto a much larger piece of fabric for reinforcement, and both the strap and reinforcement are sewn to the jersey. So, if you're looking at the back of the jersey, you'll see a small stitched rectangle with a cross through it. On the fakes, there may also be a slightly bigger rectangle around that (usually not). On the authentics, the small rectangle with the cross will be surrounded by a much larger rectangle.
Often, the people who make the fakes use the licensed replicas as their guide, so another tell-tale is if it has one of those size tags on the bottom because the authentics don't have those. Also, the bottom hem on the authentics don't have a slit in them like the replicas have.
Here's an example...
The two guys high-fiving in the middle are both wearing replica jerseys. At the bottom of their jerseys, you can see the size tag and the slit on the side of the jerseys. Authentics have neither of those things, but the fakes often do.
