I think that when someone is licking their lips their tongue is either arching up to lick the upper lip or arching to the side to lick the corners of their mouth.
Licking lips:
post #5532670, post #5514439, post #5405033
If someone just has their tongue out, it should be pointing downwards, located in the middle of the lips.
Tongue out:
post #5536528, post #5522769, post #5512948
There are certain posts where it's a mix of both, like post #5520417, where the character is almost definitely just poking their tongue out, but it looks identical to someone licking their lips.
