aside from 'venting', there's no way to accurately distinguish males from females at that size.
There are two openings in front of the anal fin in all fish; one is the vent, one is the anus. The vent, often called the ovipositor in females, is the opening from which eggs or sperm are shed.
Both males and female have genital papillae which are different shapes in males and females - in general, males have pointed genital papillae, to facilitate the deposition of sperm over the eggs, and the female ovipositors are broad and flattened to facilitate egg laying.
Venting is often a more reliable method of determining the gender of a fish by examining the proportionate size of the two openings near the anal fin. The opening closest to the anal fin is the vent ... in males the vent is roughly the same size as the anus and in females the vent looks larger than the anus.
naturally there are species specific variations to this rule and you need to look at a lot of fish to become 'conversant' in the "art".