Molly jumps in and immediately sits when she's going to pee. When she jumps in and starts digging, I know she's about to poop. Plus when she poops, she sort of stands up a little more, while when she pees, she squats a little more.
Buffy's pretty normal, she digs before and after every poop and pee.
I rarely seem to catch Willow using the box, but when I do, she does the same as Molly, jumps in and pees without digging a hole first, and then jumps out and starts scratching at the floor (I don't think she likes the feel of Swheat Scoop). I can't say what Willow does when she's about to poop. Personally I'm starting to think she doesn't poop at all, the waste simply reabsorbes into her bloodstream, lol