Outside of being a scientist with access to dna type stuff, I'm pretty sure the only way to discern the male/female cannabis plants is when they begin the "preflower" phase.

This is usually anywhere from 6 weeks to 10 weeks from when the seedling appears above ground. Usually around the same time the growth turns from symmetrical to asymmetrical. I have read that there are certain climate variables which help influence the plants one direction or the other, but I haven't tried any, nor do I know if any work.

The seedling in the pic looks healthy, you should be proud. Keep doing what you are doing.