cucumber flowers
At first Cucumber vines, Cucumis sativus, usually produce only male flowers. I'm guessing that one reason for that is to habituate pollinators to visiting the vines for nectar, before the more valuable and more energy-demanding female flowers are produced.
cucumber male flower longitudinal section (Cucumis sativus)

Above I've torn away one side of a male cucumber flower to reveal certain interesting features. One thing is that a cucumber vine's male flowers are "syngenesious." That means that their baglike, pollen-producing anthers adhere to one another by their sides, so that all together a circle of anthers forms 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 in two cases two anthers are partly merged, so really there are the usual five anthers. Actually, these are all features of the entire Squash Family, the Cucurbitaceae, to which cucumber vines belong.

In the above picture the anthers are the white-fuzz-outlined, banana-shaped items extending upward just beyond the throat of the corolla. At the right you can see an upward-pointing arrow motif, which is two of the anthers grown together. You might make out another such pair opposite it, and a single one-anthered stamen between them. The crownlike affair above the anthers is joined, twisted anther connective.

One thing to notice about the male flower is that there's nothing there resembling a cucumber. That's not the case with the female flower shown below:

female flower of cucumber, Cucumis sativus

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 others?

Unlike gourds, cucumbers are "pepos" -- fleshy, mostly edible fruits that don't split when mature. Unlike some pepos, 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.

There are some other technical details, but 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.