Unfortunately, unless they're holding, there's basically no way but to flip 'em over and vent 'em. (article on the process at
http://www.cichlid-forum.com/articles/gender.php if you're not familiar) If you have not done it, it's not at all bad to do -- the hardest thing is no worse than catching the fish out of the tank if you have lots of rocks and decor.
Labs are particularly difficult to tell -- there are lots of semi-myths about coloration and behavior, none of which have been remotely reliable in my tank.
FWIW, even though labs are relatively mellow mbuna, one of the best things I've ever done for them was to get the gender ratio to true harem proportions. They wouldn't have killed each other (most likely) but their colors and overall community got soooooo much better when I sold off the extra males. So good luck sorting out your fish!
