How can you tell stick insects are male or female?

You can't they are asexual. most of them are female an the males don't com around very often. Plus if you do get a male the female will try to fight with him to earn dominance.

ADD:

Not all species of stick insect are asexual. Many species need a mate to fertilise the eggs so males are just as common as females. A male stick insect, just like most insects, are always much smaller than the females. Females are usually broader and longer while the males are usually slender and shorter.

Also the males have a little bump on the end of their tails.