How do you tell the sex of plants apart female an male?

Most plants are neither (or both, depending on how you look at it).  The entire tree is neither male nor female.  Only in the flower is there a differentiation between the male and female sexual organs.  The pistil is the female organ; the stamen is the male organ.  
http://www.amnh.org/learn/biodiversity_counts/ident_help/Parts_Plants/parts_of_flower.htm
The pollen of a flower is equivalent to sperm sent out to fertilize the "egg" cells in the pistil of other flowers of the same species.  Once fertilized, the ovary develops into a fruit.

Male flower parts are stamen - the filaments with pollen producing bobbles (anthers) on the ends. These are usually in a ring around the female pistil. The pistil's visible end is the stigma that catches the pollen.
http://www.amnh.org/learn/biodiversity_counts/ident_help/Parts_Plants/parts_of_flower.htm


Only ~6-7% of flowering plants are a single sex. Dioecious plants have flowers of a single sex so, like animals, the plants have either male or female reproductive organs in their flowers. Female flowered plants grow pistillate flowers or female cones; while male flowered plants grow staminate flowers or male cones. Plants like  holly, hickory, garya, cottonwood,  box elder, willows, and ash trees  can be bought as either a female fruiting plant or a male nonfruiting plant.  Male flowered plants are needed for pollen production.

Gymnosperm are mostly either dioecious or monoecious with male & female cones. Cycads & Gingko are dioecious as are yew.


Monoecious plants bear both male & female single sexed flowers, or cones spaced apart on the plant. Most conifers (some pines, larches, firs, sequoia, & spruces), alder, chestnut, walnut, hickory, hazelnut, oak, beech and birch trees produce flowers or cones of both sexes on each plant. Only 5-6%  of flowering plants are monoecious.

More than 85% of flowering plants are hermaphroditic with both male and female reproductive parts in each flower. Few gymnosperm species have hermaphroditic cones.


Gnetales (Welwitschia & Ephedra species) are the only seed plants that ever have both sexes in the same bisexual cones, but this is not the rule. Most gymnosperm, including the Gnetales, are monoecious with both male & female cones or dioecious single sexed plants. Ephedra are mostly dioecious. Ephedra plants are the original source of the drug ephedrine.
http://www.amjbot.org/cgi/content/full/88/7/1326