What I got form the editorial answer is that iif you can click on the link
from the site -- ie, the game or page that originally held the link is still there -- then it's valid. (For example, I would argue that
this is proably a valid link regardless of whether it goes to an actual page or not.)
If you have to use a process_click link -- that is, if the game that originally held the link has been removed from the site -- then it's not valid.
At least, that's what I get from the response.
TNT can certainly tell whether the link is valid or not: they can see you take the link because the process_click link is on neopets.com, they know whether that link is still on the site, they can see whether the page you were on before the link contains the link or not.
To each their own. Personally, I've worked too hard on my account -- most of it before the days of cheap NP from sponsor links. I'm not willing to take the risk.
(Aside: I'm reasonably certain that how you get to a target site IS known to the target site, but I've been wrong before. Anyway, I'm not sure that's the issue here. The links you're clicking on on petpages, bookmarks, etc. aren't to the sponsor pages directly, they're process_click links on the neopets.com server. Neopets can presumably see you follow those with no problem whatsoever, since they're on the Neopets server.)