There was a human study in the 1920s by Clara Davis where they followed a group of children in self led eating habits. They offered a range of healthy foods each day and let the kids choose what they ate - generally kids fed themselves a healthy diet with appropriate portions as long as the food offered was healthy. They would even eat fish oil voluntarily and maintained good vitamin D and omega 3 levels.

Now the author never had the opportunity to try it with processed foods or junk foods, so this may not hold true when items specifically formulated to keep you eating come into play. However, it’s entirely possible that the crocodile in the image has some instinct that drives it to eat a healthy diet. Or you’re correct, maybe it is playing and using the pumpkin as a toy, but it is unusual that it would consume it then

And how do the crocs know which fruit to eat?

Instinct i guess? The digestive tract is a chem lab that analyzes nutrient contents. You've eaten it once, you get a craving once you require something of it.

With humans it's the same (cravings), you need potassium & maybe crave a banana milkshake. If you explore your craving further, eg ask yourself if you would like to drink milk or eat a banana, it might get more clear what you need (it also helps exclude carbs bcs usually we don't need them, but the self-preservation & long winters of the past usually means most humans don't have much limit on carbs, bcs why not more of them, just to be safer). It's a trained skill to some extent, especially in the modern era.

We associate nutrients with food tastes we get from our meals (which are usually a mixture of things & might even be wrong/false with ultra-processed foods).

I have no idea how this works in ancient crocks. Is it a learned (try all the foods when growing up?), observed (yo, why Silly Goose the neighbouring crock eating a jack-o'-lantern?), or "instinct" (ie only crocks that occasionally ate pumpkins survived)?
... especially given that even the current "true crocodiles" predate squash/pumpkins by 40+ million years :D.

Just ask them