can you predict which ptc site will be scam?

Tweet

lately, we see that many of ptc sites don't pay its members. it makes me feel suspicious whenever i register to a new site. i've already understood that nice words by the admin of the site ( like, for ex., "we are fair", "we are not scams like the others") are not a guarantee for a honest site. i want to ask those of you, who are many years in the ptc market: do you have any sign from the begining to predict if a site will be scam?