This from the HBG thread should help.

http://www.pondboss.com/ubb/ultimatebb.php?ubb=get_topic;f=5;t=000439;p=5

GSF
Mouth and snout: Mouth Terminal, large and oblique, with pads of small teeth on the jaws. No barbels. Body patterning, color, and scales: Back dark brown, olive, or green, sides yellow-green or blue-green, belly tan or yellow. Sides either with a more-or-less solid color, faint dark blotches or mottling, irregular faint light blue or yellowish SPOTS, and/or diffuse dark vertical bars. Sometimes 3-5 bluish lines radiating backward from underneath the eye; opercular flap dark with a light margin. Dorsal, caudal, and to a lesser extent anal fins usually darkly pigmented with faint dark blotches or light dark spots and often a light yellow/cream margin; pelvic and pectoral fins lightly pigmented to dusky. 44-51 ctenoid lateral scales. Body shape and size: Body laterally compressed and deep, somewhat elongated; oval in cross section. Typically 75-150 mm (3-6 in) TL; maximum in Wisconsin about 250 mm (10 in).

Tail, dorsal and other fins: Slightly Forked or round tail. Dorsal fin with 2 lobes, broadly joined by a membrane and appearing as one fin, the first with 9-11 spines and the second with 10-12 rays. Pelvic fins thoracic. Adipose fin absent. Anal fin with 3 spines and 9-10 rays.

GSF There are 9-12 short and thick primary gill rakers on the 1st arch.

BG
BG There are 13-16 moderately long primary gill rakers on the 1st arch.


HBG
Hybrid of green sunfish X bluegill: Notice intermediate appearance of gill rakers

These should help

GSF



GSF gill rakers



HBG


HBG (Bg x GSF) gill rakers



Pic of GSF from Bill Cody.
Note from Cody: I am not real positive that these small fish are pure green sunfish since they came from a pond that experienced 16 or more generations(yrs) of reproduction from HBG. These fish look very much like GSF to me.