Great responses guys, it's a breath of fresh air to see a topic with KO in the title that is extremely positive
How to spot a KO? Price is usually the best indicator I have found. If you see a puzzle at Meffert's for $18 and somewhere else for $6, it isn't hard to guess there.
But those are the easy cases. Sometimes a producer makes a run of puzzles based on a designer's hard work here and gives them no credit or royalties. There is no "authentic" mass produced item to compare prices, but we still consider it KO out of respect for the designer getting nothing (example: Adam Cowan's Axis cube).
Even harder are puzzles where the design isn't 100% identical but clearly derived, or perhaps the design is identical and unique, but the mechanism may be different. Hard stuff, and it takes up too much of my time considering such things.
It isn't this site's intention to say what anyone can or can't buy. We take a stance based on our respect of the designers that make this forum great, which is why I find it is worth my time to do my best to be respectful. Not everyone can spend the time to look into these things, and of course non-cubers can't be expected to have any idea.
If you care about the issue, it is usually worth spending a moment to explain the difference and suggest a site that might give them a good chance of buying something that will make you comfortable. But it isn't always easy to do so politely. I have a number of KOs that came to me as gifts, and it would have been rude to refuse them. I wouldn't buy them myself but I don't want to make someone feel bad for something they couldn't have known. If I think they will be buying more, I might mention the issue to help in the future though.
Thanks for approaching this issue is a thoughtful way,