But how can I understand if a site is sustainable?

It is not difficult. You'll need the (user) rate per click, the referral rate per click (std member) and the amount of money site charges advertisers per 1000 clicks. A simple calculation will help you to find if site is sustainable or not:

User rate (per click) + Referral rate (per click) = The amount of money site pays to its members.

Site earnings = Revenues from advertisers - The amount of money site pays to its members.

This calculation theoretically can tell if a site is sustainable. Reason is the number of self sponsored ads.

Example 1: theClickers
Site pays $0.0025 per click and $0.001 per referral click.
The amount of money site pays to its members is: $0.0035 per click (or $3.5/1000 clicks)
Charges: $0.005 per click (or $5/1000 clicks)
Site earnings =$1.5/1000 clicks
Site is sustainable.

Example 2: Zubux
When site was up, was paying pays $0.01 per click and $0.005 per referral click.
The amount of money site pays to its members was: $0.015 per click (or $15/1000 clicks)
Charged: $0.005 per click (or $5/1000 clicks)
Site earnings =-$10/1000 clicks
Site is unsustainable and that's why now is dead.
The earning formula of a PTC of course is more complicated than the simple calculation described above. But the point is, if a site charges less than pays, even if it has tons of advertisers still will lose money.
And now the question is: "How owner earns money from a site which charges less than it pays?" The answer is simple: Site is based on premium memberships and referral packs sales. A PTC can have and some banner/text ads, but this is not enough.
Unsustainable programs, are ponzi schemes and selling bot referrals in order to survive. A bot referral is not a human. But a robot which will click a few ads and then will stop. As a result, you'll not get back the money you invested.
It is not recommended to invest in such sites. Remember that bot sellers are blacklisted by advertisers. Although bots in most cases do not click ads (example), an advertiser cannot be sure for that. It is common sense that advertisers are looking for humans only. No advertisers for a PTC means, no revenue and site will close soon.
Sites losing more than $12-$14 are scams and will live for a couple of weeks only.