How early can I start topping? And how to cull out male plants early?

Hello! I popped some seeds and realized I didn't know how early I'm able to identify the sex of a plant. I've read on the forum that you have to induce flowering to be able to identify the sex. But that seemed inefficient because I'll be running a greenhouse this year and don't want to fully veg them, plant them, then have to flower for X amount of weeks then having to find out half of the greenhouse are males. I really do not want to flower then revert them to veg that just seems like I'm doing more harm then good. Any other methods I don't know about? Are we able to put them through veg for however long until they are "mature", will they show sex then?

Second question is how early can I start topping? And is there a limit to topping before it's actually doing more harm then good?

Thank you for your time

They will eventually show sex in veg, if you let them go long enough. Other than that, or access to a genetics lab, you have to put them into flower.

Common wisdom and/or Murphy's law says that males tend to be more vigorous (i.e. the most promising ones seem to end up being males). But this is unreliable, and you wouldn't want to just blindly cut down your most vigorous plants.

I take clones off each plant, number them and root them under 12/12 light I use a single CFL, they will show sex in no time and you'll have clones of the females.. It really isn't as big a challage as it sounds.... Most show in 7-14 days, doesn't stress your plants and keeps them vegging...