Koi and slider turtle pond in front of the restaurant (April 19th, 2015)

This pond with a bridge is located in front of the entrance of the restaurant. The pond holds koi (Cyprinus carpio) and slider turtles (Trachemys scripta scripta, possibly Trachemys scripta elegans and Trachemys scripta troostii).

elegans and troostii are difficult to separate and are quite variable in head, neck and shell coloration. the previous 2 photos of yellowbelly sliders are very common in my state of Georgia, USA. All 3 subspecies readily interbreed.

What is the best way to separate the pond slider subspecies, and what are the major differences between them, actually?