I spend most of my forum time in ball pythons (Limited time) but try to lurk on the kingsnake forum whenever possible.
At any rate, besides the obvious "that snake looks fat!", some other physical signs are subcutaneous (under the skin) fat deposits, which appear as raised lumps usually located in the caudal third (back third) of the body. These lumps feel soft to the touch. In addition, in very obese specimens the scales will be spread apart uniformly over the entire snake.
Behaviorally the snake will be less active than other specimens of the given species.
Finally you can expect a shorter than expected lifespan.
Hope this helps!
Scott J. Michaels DVM