The term "knockoff" refers to a replica (an imitation) of something real. It implies that it is of lesser quality. You "spot" a knockoff if you distinguish it from the real thing. E.g., a fake Rolex vs a real Rolex. So here, in this instance, it refers to the difficulty in distinguishing real love from something that is superficial. In Sex and the City (and Hollywood in general) there are many examples of love that are temporary and superficial. They are "knockoffs" of real love, which lasts.
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