How long before I can tell if plant is male or female?

I just switched the small guy over from DWC to soil and recently planted the big guy. (I had major issues with heat that I can't afford to deal with right now so back to soil for me...for now.) Small one is about a month and a half old (LONG story), and the bigger one on the right is about two weeks old now. Long story short...at what point can I tell if they are male or female? I hate that I'm going to have to kill them if they are male but...I know it's part of it.

Some strains start showing as early as week three. Many between weeks 4-5. Some strains don't show sex until they are put under a flowering light cycle.

You can take clones of the plants, root them, then throw them into flower. You'll then know which donor plants are male or female. You can also just start flowering the plants directly. You can get rid of the males and keep flowering the females, or stop flowering and let them re-veg.

I never thought of cloning. Where do I clip them, to do that?