Sometimes it seems almost impossible to tell if a site is a scam or not. Some GPT and PTC sites seem very legit and then the next thing ya know they are gone. Generally though, if you come across a site that is always having outrageous contests with $100's of dollars being paid out chances are they can't keep that up very long. Eventually it is going to catch up with them and they aren't going to be able to pay.
For PTC sites: if the site you are working is offering high amounts per clickâ€¦..5cents, 10cents, 25cents and on up. Chances are they cannot possibly keep up with that kind of outgoing cash on clicks only. I really think you are better off to stick in the penny per click and lower.
Also if the admin is absent for a long period of time with no explanation and payments are not being paid you might want to brace yourself for the inevitable.
There is a great forum you can use to check out sites that have already been established as scams. It is called GPT Boycott. There have just been 5 new sites added to the Watch List. They are At-Mails, CashKrazy, ClickSalary, DreamCatcherAds and IsabelMarco. So if you are a member of any of these sites I would be cautious.
If you are concerned about a site you are a member of you can always do a google search, trust me if there are problems going on someone has blogged about it. Try to do your research, it is not fun to lose your time and money.