Your first sign would be the low prices and the large number of similar knives. Pattern welded steel, "damasccus", used for quality knives isn't cheap and isn't that common. Any blade that looks like it may be "damascus" that is anywhere below or near $100 is "too good to be true".
Second, you'll find the same "damascus" knives on ebay and etsy and various internet sellers. You won't see them on reputable large seller sites like BHQ, KnifeCenter, etc. If these weren't junk these reputable sellers would be competing for your dollars. Instead, they have reputations to protect.
They have a very obvious "look". At least to me. Those that aren't telling to the eye can simply be tested on a fingernail, a wooden pencil, or a bit of paper.