At first, Cucumber vines, Cucumis sativus, usually produce only male flowers. Possibly a reason for that is to get pollinators accustomed to visiting the vines for nectar, before the more valuable and more energy-demanding female flowers need pollinator service. Also, there's no use have female flowers if male flowers aren't handy.
At the left, the main thing to notice about the male flower is that there's nothing there resembling a cucumber.
In that picture, one side of the flower has been removed to provide a view of the blossom's interior. The cluster of items at the top, center are stamens. The whitish, baglike items atop the stamens are anthers, in which pollen is produced. An interesting feature of the anthers is that they adhere to one another by their sides, forming a closed cylinder. Also, at first glance each male cucumber blossom appears to bear only three anthers, but a closer look, with magnification, shows that two sets of two anthers are partly merged, looking like a single anther, so really there are the usual five anthers. The crownlike affair atop the anthers is joined, twisted anther connective. Actually, these are all features of the entire Squash Family, the Cucurbitaceae, to which cucumber vines belong.
Now compare all that with the female flower shown below:
There the corolla and sepals arise atop the prickly inferior ovary, which is the future cucumber. Ovary and corolla are no more than an inch long (2.5 cm) so that ovary has a long way to go before it's a cucumber.
The big Squash Family embraces not only many kinds of squashes and cucumbers but also pumpkins, gourds, watermelons, and many other things most of us have never heard of (squirting-cucumbers and balsam-pears, for instance), so what features distinguish the cucumber species from all other members of the family?
Unlike gourds, cucumbers are "pepos" -- fleshy, mostly edible fruits that don't split when mature. Unlike some pepos, however, cucumbers contain many seeds. Unlike squash and pumpkin flowers, whose corollas are bell-shaped with grown-together corolla lobes, cucumber corolla lobes are separate from one another almost or quite to their bases.
Another distinguishing feature of cucumber vines is that their tendrils are "simple" -- instead of being branched as in some genera. Tendrils of vines of pumpkins and squash, for instance, are branched.