Young dioecious plants, how is gender discovered before old enough to flower?

About 5 years ago I planted two kiwi vines.  I bought and planted one labeled FEMALE and one labeled MALE.  
As luck would have it, they finally both bloomed this year, and both are  clearly FEMALE.

How do nurseries even know if a vine/tree is one gender or the other before they have obvious sex characteristics like flowers?

I've bought a third kiwi vine, supposedly male, but I hate to plant it just to find out in 4-5 years that it is also female.

Usually I think these plants are grown from cuttings of already matured plants.  I have three pistachio seedlings in my yard right now and am hoping to find in a few years that they are a mix of male and female.  If that isn't the case I will order one tree of the missing gender from a nursery. That tree will have been produced from a cutting.