What does a healthy weight look/feel like?

I am far more used to judging a healthy weight in cats and dogs.

I realize that even after Malfoy's 8 month fast, he had to have lost SOME weight - he was 6 months into it when I got him, and his previous owner never weighed him.

Understanding that many snakes end up overweight, I wamt to get an idea of what a healthy weight feels like for a ball python, or what to look for. Is it normal for me to easily be able to distinguish his spine both visually and by touch? Should I see his skin wrinkled in places when he is in a ball? Other things I should look for?