With dwarf gouramis, if they are the regular ones, the color difference is considerable and you can easily tell because the male is very much more colorful than the female. With the newer varieties, they seem to only be selling the males, so you might not be able to get a female.
Female dwarf gouramis are silvery grey, males are brightly coloured with blue and orange vertical stripes - very close together. The males shimmer in the light.