If you are talking about yellow labs (Labidochromis caeruleus) then they are extremely difficult to sex without venting. Egg spots can be an indicator, usually the more egg spots on the anal fin the more likely it is a male - but that is not a perfect science. Also, a 20g with 2 adult yellow labs is pushing it. What is the footprint of the 20g? It would be better to do a 20g long for the extra width (30" over 24"). This can be done, but it is not recommended because once you add some rocks for the fish (yellow labs are mbuna and therefore like/need rocks for hiding spots) and other decor you don't really have that much water. The biggest thing is to keep up with your water changes because your parameters will rise very quickly (talking nitrates once the tank has cycled).
One thing I would suggest is to try out cichlid stones (
http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=21383) (they can be found other places online as well, and possibly even your LFS). I have never used these, so I can't really recommend them, but since they are hollow you won't lose as much water as real rocks. This would increase your chances of success.