[SpasticPlastic]

1. The box has the wear marks, which is good.
2. The Nickelodeon logo looks to be towards the bottom of the orange rectangle, also good. Though, the orange does look a little dark, but that could just be the picture.
3. There are no seam lines on the arms, which is a big tell for a knockoff.
4. The spots on their skin don't seem to be too dark or too big and also have a "softer" appearance, meaning the edges of the spots are not too crisp. Very good.
5. The chest shell part has a duller almost tan paint with dark wash over it, especially towards the edges. The knockoffs I've seen are painted a brighter yellow with no shading.
6. On some knockoffs, the "nose" part of the turtles' heads are painted quite a bit lighter than the rest of their face, or have a thicker wash or dry brushing of a yellowish colour. These look like their paint is consistent and not too heavy-handed.
7. And, finally, the holes in the plastic tray where their arms go through are cut smoothly. The holes on knockoffs are rough and chewed up looking, sometimes with tears or rips.

Michaelangelo's eyes are a bit wonky, but I've seen legit Turtles from NECA with that problem. They are hand painted, so there is room for error there.

All in all, I would say that I, personally, would buy this set with about 95% confidence that it is authentic. Bootleggers are crafty scumbags, so there's always a slight chance they upped their game and fixed some of the "knockoff tells", but these really do look real.

Hope that helps.