A snake will not get parasite randomly. they usually come from the wild. CB snakes dont usually carry them and if they do they rarely reoccur once treated.
Runny, and very foul smelling fecal matter is a sign of parasites. so is regurgitation, and lack of appetite for extended periods. All new snakes should have a fecal float done by a vet its cheap and it tells you right away if they have a parasite or not. and if it has a clean bill of health after the fecal is examined then your chances of hetting them after the fact are very slim.
As for mites. you will know if you have them and they are easily treated with a few different products out there. i prefer Provent-A-Might as it works wonders but, it can be rather poisonous to your snake and even other pets depending what you have so read the direction very carefully.