Sites that probably are scam have some of this caracterisics:
*They don't have a forum
*They pay up to 5 cents per click
*Have a payments proof in .html
*They have very low prices for advertisers (less than 1 dollar per 10,000 clicks) [not always, like lysvalley ptc]
Richptc and 1 dollar ptc are examples of this kind of SCAM!