Three things to look out for in a new snake: diseases, external parasites, internal parasites.
Your new snake may have been recently exposed to any number of diseases, especially if it just came through a pet store or importer. My vet was just telling me how common IBD is in boa constrictors. Apparently they can live for a long time with it but a python will die quickly. If you keep your new snake in isolation for long enough the hope is that it will either die from or get over any diseases and at some point after it has been healthy for long enough you can introduce it to the rest of your snakes without worrying about it introducing new diseases. I'm not sure what the recommended quarantine period is but I'm thinking 3 to 6 months.
External parasites (ticks and mites) are thought to be one of the main ways that diseases like IBD are spread through collections. Watch the water bowl for drowned mites or check around the eye sockets (at least I used to see them there in Burmese pythons, I've been lucky enough not to have had mites since I switched to ball pythons). Although ticks are often visible they can apparently go undetected sometimes. It's probably best to just go ahead and treat all new snakes for mites and ticks while they are in quarantine and then keep and eye out to be sure you got them all.
Internal parasites requires a fecal exam to identify. Find a good reptile vet and see how they would like to do these. I bring my vet fresh (still wet) samples (the dark stuff, not the white) in double zip lock bags. As long as you keep them cool but don't freeze them or let them dry out they can keep over the weekend fine. After identifying the parasites your vet can prescribe the appropriate treatments. Apparently almost all wild caught ball pythons have some type of parasite, I'm not sure how common they are in the wild breed/captive hatched, I suppose it depends on the quality of water and cleanliness of the cages they are kept in before they reach you. I think a lot of people with large collections just go ahead and treat everything for everything without testing but I don't get enough new animals to justify keeping fresh meds on hand so will probably continue to take samples to my vet. I recently discovered that an animal I got two years ago from a pet store that was doing very well had two types of internal parasites but presumably she and her mates will do even better cleaned up. Parasites are just an extra stress that a shy stressed captive ball python doesn’t need and can sometimes overwhelm and kill them.