This is a very difficult thing. I guess if you have one and don't know, you can't worry about it. Often the most common way to tell is they are produced in a colour the company NEVER produced. Otherwise, I'm not sure. If the colour isn't true all the way through the resin, especially in the joints, air bubbles that aren't mentioned and very noticeable, though I have had an original with similar faults twice, so that's not the absolute sign.
I wouldn't buy one knowingly.