When talking about PTCs or PTRs, a scam is a fake program, which usually imitates or copies the site look from famouse sites. These fake programs look and work the same as any legitimate program, except that you will not be paid. Scam sites usually are new and often offer some outrageouse payout. Scam sites may have working ads, and may actually give ads to the paying customer. But they don't pay the viewers of the ads. They will post false proof of payment and have forums where fake people praise the program. These sites usually last 1 month, at which point they will change the name, the layout, and the site name to start anew.
There are many sites which try to keep a list of scam sites, but the constant changing of names and designs, makes it hard to keep the list up to date with the newest scams. It is therefor adwised that you do not spend any money or put to much effort into any new program that is not part of a trusted company (such as bux.to). I would advise you only to join new programs when they are opened by trusted companies. Do not join a program just because it claims to be affiliated with a big shot. Do your research and check the site the new program is supposedly affiliated with, to check wether it is affiliated. All sites that I advertise on this website are all trusted sites, or affiliated with trusted sites unless I label them otherwise.

Below are some sites concerning this matter:
uronlinebiz - this site has reviews of commonly used PTC programs and also ranks them. This site will have news about suspicous PTCs. If you are a member to this site, you will also have access to the scam list.