Regular Seed Question

You can’t sex a regular seeds before it has been flipped to flowering cycle.

I don't know what "best" would mean in your situation. Could be sending a leaf sample to a lab. Doing it fast yourself goes like this: Give your mature plants 6 hours of light to sex them. The increased scotoperiod makes them express their sex very fast, and gives them just enough light to cover their basic needs. Right on the edge though so no less than 6 hours of light. Some will show within 7 days if you have good eyes or magnification to look through. Others will take a few more days. By day 12 you should know which are male and which are female. Males tend to show first, but some females get excitied about the darkness they love and want to join the party early.