I can't see the whole thing but these are some red flags:
Poor or just outright wrong stitching, such as it being crooked or stitching going in places it really doesn't need to.
If there is anything at all left of the tag, look for a blurry Pokemon Center logo.
Fabric often feels rougher, cheap, or is an older, material, which shouldn't be the case. These things are made to last.
Check for empty parts that may not be filled in with stuffing, this is a large red flag, as the Pokemon Center usually doesn't do that.
Check if the headpiece is sewn on, or just glued. It looks a little off in the picture.
Also, the Pokevault website itself says the plush cannot stand on its own, so try to stand it up.
Hope this helps you!
Reference Plush:
https://pokevault.com/PokemonPlushToy/GlaceonPokedoll