[corobo]

It's an account-wide setting to allow the monetize option. From there it's up to the account owner to enable/disable per video they upload [1]. There are other things in place such as the ContentID matching which will disable ads until you prove you're allowed to use the content if it matches anything but generally speaking it's up to whoever owns the channel you're viewing videos on. They would almost definitely know they have ads enabled - there's even a green dollar sign next to the video in the manager if ads are enabled

Advertising-wise you can target pretty much anything/anyone Google knows really - You could very easily (It's one of the easiest options to configure even) target people in a specific country. There's more advanced stuff such as re-targeting people who have already seen the ad/content specifically too. I wouldn't be surprised if there was a way to target a specific channel or at least whatever common content is in each of the videos (the most notable mine name involved, for example).

[1] http://i.imgy.org/5f/49/adoptions.png