How can you distinguish the dorsal and ventral sides of an earthworm?

Yes

Explanation:

The earthworm has a rounded dorsal (back) surface and a flatter ventral (belly) surface. Usually the dorsal surface is darker than the ventral surface (though sometimes this is obscured in the preservation process).

The ventral side is lighter than the dorsal side and more pale and kind of flattened unlike the dorsal side which is darker and round.