I think that the spy dynamic is a very fickle one. A spy should serve as a situational killer, distracting enemies. You can, if you're paranoid, become immune to spies, but it's very inefficient in combat. Like a lot of classes, the spy is most effective when he has no aggro. Gameplay wise, the ideal spy-checking method would simple be looking at them for a few seconds, or some way of measuring whether you're suspicious and aware of their presence. Cloaked spies shouldn't be able to be found, far from an incredibly stupid spy. Of course, the best thing to do here would be a change whereby every single melee weapon un-disguises a spy, and drains/prevents cloaking. That way, a found spy can be tagged and hunted. but that's just my opinion.