There's a certain caliber of "mineral shop" that tends to be full of crystals cut into "natural" shapes and trilobites with every feature rolled into one (eye stalks, antennae, horns, etc). They tend to be crazy expensive, obviously fake, and sold primarily to looky-loos who would never otherwise buy a trilobite fossil. Go to a gem and mineral show, they're nowhere. Go to Santa Monica, they're all you see.
Trilobytes are pretty cool, so it would make sense to rip off. If I'm honest, I probably would have a hard time figuring fake from real if you put them in front of me. I think that's where reputable shops come in. There's a really cool non-antique shop that Dala and I have visited a few times that has local fossils in it and for a hell of a price too. I'm fairly confident they're legit though, because they also sell things such as bug specimens and the like. While they certainly aren't selling to my demographic, they also don't feel kitchsy either.