Since most PTC site is scam, it is very difficult to find out some genuine ones. But it is not as difficult as it may seem. You just need to understand some basic features of a scam site. Then use your intelligence. You will understand it easily.
Remember, every genuine PTC site has forum so that members can join and share their experiences with other members. If you are planning to join a particular site, you need to find out if they have any forum or not. Because if they have a forum, you can join and ask other members about their experiences.
Genuine PTC sites wouldn’t bother to have a forum to help its members share good experiences. Only scam sites won’t have any forum for discussions with other members. Check how long the site has been online and the minimum amount you have to earn to be eligible to get paid.
Search Google if you find out any review of the site. Search Google Image for screenshots of payments. Many members become very happy when they get paid from a PTC site. Genuine PTC sites would ask you to post your payment screenshot in their forums so that other members understand they are genuine. These screenshots get indexed by Google Image.
So Searching Google Image for some payment proofs would be a good idea. Check if the site is changing its terms and conditions or payout policy frequently. Check if they respond to your query if you contact them. How long it takes to be contacted.