You can't tell by the sound of the engine? How far down your foot is if on a flat road? How fast the cars go past you? I can tell as a passenger if we are speeding even a little bit.
Maybe some people pick up on this better? I only check my speedo for a second every 30 seconds or so. I manage not to accidentally put my foot down hard in that time. You know you're supposed to keep checking mirrors as well don't you?
The logic that you are not watching the road if you do these things is crazy to me. They are part of driving!