How do you identify what is really antique or something made to look antique?

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When you are looking in antique shops, flea markets and second hand stores how do you that what you are looking at is really and antique. Is it antique or just make to look like antique? Sometimes I cannot tell the difference. I don't buy the antiques because they are too expensive anyway but wish i could but I would really want to know the difference first. Can you tell the difference? What do you look for?