I don’t think a kitten can tell. A fleshy warm bump surrounded by hair is what they like.

Even adult cats are known to nurse on fuzzy blankets.

Here is a video about two orphan kittens. One nursed on the male kittens genitals and it almost killed him.

Very serious business.

Spoilers he didn’t die but came very close to it.

https://youtu.be/4HoppAb3Md4