Scott, till joining this site, the only way I could tell was when I gutted them while cleaning. Now, I couldn't tell that way because I've started filleting my gills after seeing a video of it by another member. Never even braek the gut sack anymore. But I have learned this and went fishing yesterday just to put some pics together for you on this subject. Hang around, you can learn a lot from this site.
Bruce Conello, the founder of this great site has a more thorough picture presentation, with better pics somewhere, perhaps on the Pond Boss website. Since you are growing your own, you may want to check out that site anyway. There is a link on the main page here. Hope this helps. If nothing else, just remember, boy gills are prettier than girls gills!