How To Choose a Healthy Snake
Only purchase snakes from a pet shop that has clean cages.
Check for wounds on the snakes body.
Check the snakes belly for sores. This would indicate scale rot from being kept in damp conditions.
Check for discharge from the snakes nostrils and mouth. This would indicate a respiratory infection.
Check the mouth for sores or discharge that would indicate mouth rot.
Check for mites crawling on the snake. Look out for silver specks on the snake this indicates a mite infestation.
Check the snake for ticks. Note: It is highly unlikely you are going to see ticks on captive bred snakes.
Check the vent to see if there is any fecal material caked around it.
Run your finger down the back of the snake, you should not be able to feel the snakes spine if it has been feeding properly.
Feel along each side of the snake. You should not be able to feel its ribs if it has been feeding properly.
Run your fingers along the belly of the snake. If the belly caves in when you feel it the snake has not been feeding.
Get to know how a healthy individual of the snake species you wish to keep looks and behaves. So you can make comparisons with the snake you want to buy.
Ask the seller if the snake has been feeding and what it is eating. Most breeders will state if their snakes are eating any prey items other than mice. For example breeders should state if their baby White Lipped Tree Vipers and Horned Adders are lizard feeders or if they are pinkie feeders.
Finally take your snake to a reptile vet for a post purchase check up.