You can't know that others are altruistic because you can't know the contents of other minds, but you can still know that altruism exists.

To know that a person is acting altruistically we must be able to know what is motivating them. But we cannot know what motivates others because we only have access to our own thoughts. What many conclude from this is that altruism doesn't exist. By this rationale there is also no selfishness, but this doesn't prevent altruism deniers from attributing every human behavior to self-interest. An altruism denier knows that everyone is ultimately self-interested! After all, if they weren't, then they would be forced to feel guilty about how they live their lives.

Altruism exists, and it is extremely easy to know that it is real. All you have to do is act altruistically and introspectively. It's that simple. Of course, this is assuming that introspection grants us access to our own thoughts. If you think that the subconscious prevents us from knowing what we believe then it's hopeless. Of course, if you believe that we can't know what we believe then you wouldn't be able to believe that either... or that either... or that either... or that either... ...