Judge by blossoming time. Male blossoms are always produced before female blossoms, to ensure adequate pollen to fertilize the female blossoms.
Examine the organs in the middle of each blossom. Those that come to a point are male blossoms; those that form a ring are female blossoms.
Look at the stem of the blossom. If it is thin and woody, the blossom is male. If the stem is thick and bulbous, it is female. The bulbous portion is the ovary, where the pumpkin is beginning to develop.
Count the blossoms, if you are unable to recall which characteristics correspond to male and female blossoms. The differences are easily recognized, with male blossoms outnumbering female blossoms by approximately 10 to 1.