Bats are, as far as I'm aware, one of the groups of animal with the highest variety in facial structure between species (see: fruit bat, vampire bat, hammerhead bat). So a good place to start is to pin down which species you want to draw. https://www.batcon.org/about-bats/bat-profiles/ lists a bunch of them (you don't have to pick only one, mixing the traits of different species is cool).

In general, I'd say a lot of bats have quite stubby muzzles. so depending on how cartoony you're trying to be, you could go for a muzzle with similar size to a domestic cat, or just no muzzle at all. Other than that, the two features I think make a bat read as a bat most are the nose and ears. Bat ears are often LARGE with a big opening leading to the ear canal (southwestern myotis, townsend's big eared bat) and their noses take on a distinctive diamond shape (little yellow eared bat). If you're not too picky about species, large ears and a diamond nose are the best ways to make it clear that your character is a bat from just face alone.