This semester, I'm taking a seminar on Comparative Psychology, and have found myself to be repeatedly frustrated with an overwhelming majority of the studies we have read. Maybe I'm becoming too opinionated of a reader, but it seems to me that two fundamental things are missing from a lot of the studies. When trying to understand whether or not non-human subjects possess specific psychological abilities (e.g. theory of mind, self-recognition, reciprocal altruism), you must keep two things in mind. First, animals may not perform the behaviors we are looking for without adequate motivation to do so. Secondly, completely novel situations may provoke an animal to react in a way in which they would not customarily react. Therefore, it is important to provide situations that are somewhat familiar to ones they might encounter in their day-to-day lives in their natural, environment to which their species has adapted (unless, of course, using a novel situation removes a bias in the experiment).
Basically, in order to conclude that a subject with whom you cannot communicate with does not possess specific mental abilities, you must design an experiment that will do its best to elicit it. Only then can you conclude that the subject does or does not have such capabilities.