Spot = *see 雲小泉's answer Phony = An impostor; Something not genuine; A fake. So, "to spot a phony" means = to recognize a fake

spot = see; recognize When you notice something small among a lot of other things, then you use the word "spot".

In this context, there are two types of warning messages: real warning messages and fake warning messages. "Phony" is normally an adjective that means "fake, forgery, not real, not genuine". But it can also be used as a noun when the reader already knows what kind of thing you are talking about. In this sentence, if you know what a real warning message looks like, then it will be easier to spot a phony warning message. But we remove "warning message" because it is already in the sentence once. We use the same technique in similar phrases like "recognize a fake" or "identify a forgery". We use the same language to talk about fake money, forged I.D. cards, fake driving licenses, imitation clothing brands, etc.