What kind of checkerboards? filamentosus? If so, the sexes will be apparent by about 1.5", and the males will develop lyretails and more color to the fins. Females have shorter and colorless fins, unless she's breeding/ready to breed, and then she gets red pelvic fins. If they are small, you may not be able to sex them.
Also, all of the checkerboards require soft water, especially filamentosus. The maculatus you see pictured don't require as acid a pH, but they still like a very low hardness. Mine are in the following conditions: pH 6.10, dKH <1, conductivity 150/TDS 80ppm. Outside of breeding and during growout they do fine at a pH in the low 7's, but the hardness is still low.