Determined via trial and error. Basically we use a helper like a Chrome plugin (Whitelist assistant from DNSThingy is one of them.) We visit the site in question and try to find the correct sites the whitelist in order to get functionality.
Sites like YouTube are constantly on the move with thousands of programmers to thwart any adblockers, and we're just a team of volunteers on spare time trying to be the David to their Goliath. Community help would be awesome and we have a section for community submissions to blocklists. (Commonly Whitelisted Domains)