Avoid this Websites.

1) with free host/free domains. It is common sense, if an owner can't spend some money and get a good host and his own .com/.net domain, that he'll not be able to pay his members for long time. A domain costs ~$10-15/year, only!
Hypothetical examples of free hosted sites: www.yoursite.freehost.com
Free domains: co.cc, .tk, co.nr etc.

2) with default design. There are many PTC scripts (yourownbux, securebux, bux_to_clone) and each one has default design, with predefined colors, pictures etc. The same rule applies and here: If an owner can't afford a new and unique design, how will he pay in long term? Also this can be considered and as sign that owner doesn't care about his site. Such sites can be easily spotted.

3) using unmodified YOB, Gen2 or Securebux script. It is well known that YOB script has lots of backdoors placed by the creators/distributors. YOB was the "No1 Pirated Script - 2008". Same and for securebux script. Gen2 script creator/owner (also creator and of securebux) is a known scammer. He was owner of ubux, buxor and bux1 and scammed thousands members. Gen2 now is open source (= free). Expect hundreds of new scams. Seems that creator finished his job; plenty backdoors are waiting unsuspected members and candidate admins...
These scripts need modifications (find and patch all hidden backdoors). In case an unmodified script is being used by a PTC, even if owner has good intentions, hackers can use these backdoors and within seconds site will be hacked...

4) without forums/support. This is proof that owner does not care at all and doesn't want to communicate with members. Installing a forum is very easy. For example smf forum software (which is free to use) can be installed within minutes. However if a site has forum, this does not mean that it is not scam: The biggest scams had/have forums in most cases. Scam sites with forum usually have an abandoned or full of complaints forum. And this can help a lot.

5) with "instant" payments. Some sites, with announcements will try to convince that will pay instantly "soon". The best example is earn.nu (now seems to be dead), a site which was owned by a known scammer: Instant payments was just a fake promise and only for premium (!) members. Invest to get paid instantly? The answer is "No, thank you". Don't believe these announcements. And extra10/earn3 former owner announced that he might implemented instant payments, but finally sold both sites, scammed many members and started a new one (buxout.com). Another example is crewbux, which is promising "instant" payments for honest/elite members (= members who invested hundreds of dollars), only. But the instant payment script is not ready yet (of course).

6) with default description/TOS/FAQ. Scripts also include and a default description/TOS and FAQ. Some sites for example are paying $0.005 per ref click, but in the TOS you may find that are "paying" $0.01/ref click!
If a site does not edit the TOS/FAQ at all, this is proof of a lazy admin who doesn't care about his site and probably has no intention to pay.

Such sites will scam soon (even and after one week!) and are very dangerous: Owners or hackers may try to hack your accounts in other sites using the password you have for their site. Remember, use unique passwords! Also they may try to sell their database (= your email address) to spammers.