My kitty Pants has EPI. It's diagnosed through blood testing, specifically for Trypsin-like Immunoreactivity. Cats with EPI also often have low B12. The pancreatic insufficiency is treated by adding enzymes to the kitty's food. Any accompanying B12 deficiency is treated with B12 shots, first weekly, then going to biweekly or monthly if the levels are OK.
For most cats, the treatment is super effective, but that hasn't really been the case for Pants. She didn't really have bad litter box symptoms, her problem was weight loss, and she hasn't gained back weight. She's got a bunch of things going on, though--poorly controlled diabetes, EPI, IBD, and food allergies--so her situation is likely complicated by her other issues.
If you do end up needing to use enzymes, I've learned that stinky food and/or adding flavor (like with FortiFlora) helps mask the taste if your kitty is offended by it. The packaging on the enzymes tells you to wait 20 minutes to feed so it has time to work; one of my vets told me I didn't need to do that, and I learned the hard way that for Pants, I really do need to wait. She got burns on her mouth when I didn't
