We have a lot of experience touching things with our fingers (in daily life) and tongues (more when we're younger), and can extrapolate from this experience.
In my own experience, this extrapolation is overconfident - I tested licking a few random items, and while my expectation was close to reality, I was always just a little off for the truly novel items. Of course for something gross, most people won't (and shouldn't!) test their expectation, so they go on with the confirmation bias that they got it right.
First example that comes to mind is that the roof of my mouth feels much smoother to my tongue than it does to my fingers.
In my own experience, this extrapolation is overconfident - I tested licking a few random items, and while my expectation was close to reality, I was always just a little off for the truly novel items. Of course for something gross, most people won't (and shouldn't!) test their expectation, so they go on with the confirmation bias that they got it right.
First example that comes to mind is that the roof of my mouth feels much smoother to my tongue than it does to my fingers.