Basically, DNS requests for those websites are redirected to a government controlled webserver.
What other techniques are there (besides checking every site on the internet) to try and uncover whether sites are blocked in Belgium?
1. Legal (most ineffective)
2. Pursuade an employee of a Belgian ISP (they must have the list)
3. Brute forcing the "government controlled" webserver and seeing what it responds to (might respond to everything equally though)
4. Crack into Belgian ISP or government anywhere where the list is compiled or actually applied. (Not recommended)
Up to about 8 characters a full brute-force is kinda feasible. I suspect that some blocked websites might very well have more than 8 characters, so it truly doesn't work. Using this index (highly questionable) it claims 40billion pages, which is actually kind of doable. Problem is obtaining that list, which is not quite doable. (And maybe your ISP claiming you're operating outside normal parameters)