United Kingdom 
how do you tell if a apple tree is male or female
28 Mar, 2011
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You don't, because they are not dioecious (i.e. only one sex).
The reason that you generally need two or more trees to get a decent set of fruit is that many/most varieties are self-infertile, which means that they cannot fertilise their flowers with their own pollen.
28 Mar, 2011