According to an article on caribou sex identification by the Alaska Department of Fish and Game
Only the white rump without a [black] vulva patch or the presence of a penis
sheath should be used for positive identification [of a male caribou].
The article says that both sexes have antlers, and there is significant overlap between young bulls and mature cows in antler characteristics. Antlers are not useful for identifying males.
Nor can you use testicles as a mean of ID-ing a male. Testicles on a bull look like the udder on a cow.
The direction of the urine stream will help. The urine stream from a cow goes backwards; from a bull, forwards.
Whether you need binoculars to spot the penis sheath or the absence of the black vulva spot on the white rump is not absolutely clear from this article. The article advises you to stay 200 or 300 yards away so as not to spook the animals; at this distance, you will need binoculars. But the article assumes that you are hunting.