There are many sites that offer insight on whether a particular PTC/GPT site is legit or not.
I use PTC Investigations as a good source of infor about sites that have been around for a while. (Just use google to find it.) It doesn't help much on brand new sites since it generally take 3 months for them to investigate and try the site and by that time, the bad sites are already gone.
For existing sites, I've also found that I can use google: I enter the name of the site and the word scam and I will frequently get several websites (forums, mylot, PTC investigations and many others) that offer insight about the site and whether people are happy with it or not. If the site is brand new, you'll probably just get the site's website.
Another source of information that I use is Whois. To find it, I just google "Whois name of site" and I'll get a couple of sites that provide domain information about the site.
Of particular interest is the name of the owner, his email address and location.
That doesn't help you much unless you know whether that person is reliable or a scammer. But occasionally, I have gotten a response from the owner when I emailed him when I couldn't get a response via the site directly.
If you are asking this question about the "offer" websites, then look through the entries in the Forum (there is a section about offers) that will give you some insight on which offers work and those that are more difficult. Suggest you read through the Beginner's Guide (top entry in the support section)
While you'll doing this background work on the regular offers, try the $.15 click in the daily section of offers. Sign up for the paid email (worth $.02) and check for video ads in the Special tasks section. The Daily section also includes all of the surveys that you can do once every 24 hrs (do= complete). If you get disqualified for a survey, you can continue to try until you actually get the completion/congradulation window. Leave that window open for at least 10 minutes (or until you see your credit on your Account page)
There is a general recommendation that you clear your cookies and temporary files before you try any regular offer or survey.