The unfortunate answer is that it knows through long forgotten experience... Anything a toddler sees goes in the mouth.

I'm pretty sure it's my brain extrapolating from that baby experience and combining it with finger sensations. I didn't have the opportunity to lick a basketball or (US) football but my tongue can surmise the difference in the bumps because my fingers know.

Your ear knows how it will feel too, and your pinky toe, and your nose...

False. I have no idea what licking lava would feel like.

Not yet. Wanna try together?

Suuuuure, buddy.

Hi, my name's Lava

Ever eaten Hot Pockets?

Unfortunate? That's an important part of learning as a toddler.

Hotdogs/brats with cheese in them are in the same category.

Isn't this the same for hands?

… and my penis?

Easy there, Vance

And my ass

Yeah weirdly (or maybe not) when we get down to it too every sense is a sense of touch. Your ears touch the vibrations in the air, your nose touches the particles in the air, your eyes feel the touch of the light that hits them, skin is self explanatory, and then your tongue, which is touching on a very miniscule level while checking for all the chemicals and seeing if your nose is confirming that the touch is similar to the particles

Some toddlers have helicopter parents who never let them put anything in their mouth. Will they be stunted in their growth and development and sensory skills?

A healthy anus can tell the difference between solids, liquids, and gasses.

Sucking on couches & table legs & footballs. Toddlers have a lot going on at school.

"Is your Hot Pocket cold in the middle?"
"It's frozen, but it can be served boiling-lava hot."

Hot Pockets!