Good question @misshugo.
Actually yes it is possible to tell before flowering, but not always.

The plants will show "pre-sex" sometimes.
This is when very tiny indicators of pollen sacks, (male) or a few (female) white hairy looking pistils shoot out.

A presex indicator is usually a very reliable indicator.
Sometimes hermaphrodites can happen, but that's another topic.. usually induced by stress or genetic issues.

I saw some female presex signs on one of them already but lost track of what plant it was. I know I have at least one in there, but there will probably be 2 or 3.