We don't know exactly what's going on in someone else's mind, but we can take a guess based on their behavior. That goes for primitive species like worms with few neurons, up through animals with many neurons, to humans, who have taken the concept of "mind" a lot further. Yet we still don't know where to draw the line on which animals have "minds" as we think of them. Heck, we can't even define "mind" as opposed to sentience, self-awareness, and consciousness.
Humans, with the most complex of all brains, have managed to use our collection of neurons to develop and understand higher concepts like empathy, morality, art, fiction, prediction, language and math, long-term planning, and civilization itself. We've become pretty good at telling others what's on our minds, although we still cannot totally experience the way someone else thinks. Kurzgesagt gives us a brief tour of how brains differ between species and how more complex brains work. There's an ad from 4:44 to 6:00. The video ends at 10:42.