How to spot the difference?

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I have been joining a lot of PTC sites lately. Some exchange referrals and some are a sponsored site from one of my forums. And it really puzzles me if the PTC site that I've joined is a scam or not. What are the hints and tips of seeing a scam site from a legit one? What sites do you think is the best PTC site of all, are there list of legit ones?