Any tips on how we can know?...
A red flag is a maker or seller based in Red China. Another is Russia or other Eastern European countries. Also SE Asia. On any platform.
This is not 100%, of course, and I expect replies from some of you such as "So-and-so is in the PRC and he produces only his original work". And there are plenty of pirates in Western Europe and the US. It's a generalization, but that means that it is generally true. Country of origin is definitely one thing to look for.
You'll learn as you go. If you shop on the trading sites based in Asia, you'll likely see more pirate work, too, though there is plenty of such product on eBay, too.
Look for sales or auction post that use the artwork from a known commercial producer. Is the item a resale of a Mitch's Models figure, for example? Or is it a sale that does not reference the maker at all, but uses the original box art?
Is the piece in resin but you have seen the original in white metal? Or vice versa-white metal copy of a piece you know was originally sold in resin? (That does happen. I have seen Russian white metal copies of resin kits on eBay.)
And despite what some people, especially sculptors and other commercial producers may say, poor quality is NOT a sign of a pirated piece. That may have been true 50, 60 years ago, when figures were in white metal, and to copy them, you had to make a mold and cast in white metal. Today's pirate copies are as good as the original. I have seen them.
Really the only way to know is to see a lot of original pieces, and to ask questions as you have done. Piracy is a big deal, and there are other cultures where our concept of intellectual property is not as much of a value as it is here in the West (where there are enough people who disregard it).
Hope that helps you!
Best regards.
Brad