I know what's on your mind - I know what you are thinking. I know what's in your head - I know what you are thinking. I know what's in your mind - I know the total content of your brain (Doesn't really make sense. However, you can say "I know everything that is in your mind.". This means that no matter what bit of knowledge you would say, I already know it, I am smarter, or more experienced than you). I know what's on your head - I know what hat or other head gear you are wearing