This plugin is an excellent resource for determining the general reliability of a news source..
It should be noted though that the plugin does not claim, nor could it reasonably claim to verifiy any news source as 100% reliable.

One further observation, almost every negative review comes from someone who tried to use this to verify their pre-existing assumptions about a news source, and dismisses the validity of the plugin because it does not confirm their existing presumptions - curiously enough the very target audience most biased sources prefer, imagine that.