How do we know that plants and similar don't feel pain? As far as I see it plants just don't act like they're in pain, but that doesn't mean they aren't. They could just be very stoic about it.

You might just as easily wonder how you know that other people do feel pain. Both questions are instances of what philosophers often call "the problem of other minds." I believe that other people have psychological states (thoughts, beliefs, sensations, etc.) and that things like rocks and plants do not. How do I know this? It seems on the face of it that I cannot know that someone (other than myself) is in pain in the same way that I can know that the table in front of me is brown; I cannot directly observe the pain of another person. So presumably I must know that someone is in pain on the basis of something I can observe: his or her behavior. I see a batter get hit by a pitch. He falls to the ground, grimaces and writhes around. From this behavior, I infer that the batter is in pain. In a similar vein, when a plant does not exhibit any of that kind of behavior (or anything that could be understood as behavior at all), I conclude that the plant does not feel pain (or anything else, for that...