Determining if a zucchini blossom is male or female. Examine the blossom stems, the male blossom has a long, slender stem, while the female blossom is on a short, bulbous stem which is the ovary (unfertilized zucchini).
Only the male blossom has a stamen in the center of the flower. The pollen on the stamen pollinates the female plant. When harvesting zucchini blossoms only harvest the male blossoms making sure to leave a few to pollinate the female blossoms which should not disturb actual zucchini production.
First harvest of the morning remove those blossoms that are open, return to the crop in the afternoon to check for new opened blossoms (removing only those of male zucchini). It is much easier to fill an open blossom rather then trying to pry open a closed blossom and risk tearing its delicate leaves.