Compare the two types of fish in this photo. The only fish in the photo that are sexually mature fish are the largest GSF (lower right) and possibly the largest BG.
As mentioned above, GSF tend to be a more narrow bodied fish. Notice the mouth of the GSF is noticably larger than that of the BG. Look for the back or farthest most part of the jaw. Jaw mandible of GSF extends at least to a line vertical to the eye. BG jaw mandible falls quite a bit short of reaching the front of the eye. Also GSF will always have at least one or several emerald streaks or lines on the gill cover and the bgill gill cover is just about always plain on fish the size you are talking about catching - never any emerald streaks. HyBG will have coloration and markings that are inbetween these two fish and often one type of emerald streak. See some of the referenced photos. Coloration and markings on these fish are fairly normal, although expect to see somewhat lighter or darker coloration or markings when the fish come from light or dark waters.
BE aware that there could be other types or species of sunfishes in the pond you are sampling.
