Knowing whether your beetles are male or female matters if you want to breed them, but also just for understanding what you've got. Males and females of many species look and behave differently, and in some cases have very different adult lifespans. The problem is that sexing beetles ranges from dead easy to nearly impossible, depending on the species and the life stage.
The easy ones: horned species
If you're keeping rhinoceros beetles (Dynastes, Allomyrina, Megasoma), sexing adults is straightforward. Males have horns. Females don't. That's it.
In Dynastes hercules, the difference is dramatic. Males develop a long thoracic horn and a shorter cephalic horn that together can double the beetle's total length. Females are plain, slightly smaller, and hornless. With Trypoxylus dichotomus (Japanese rhinoceros beetle), males have a forked cephalic horn. In both cases, you can tell them apart at a glance once they've eclosed.
There's a catch, though. Some male rhinoceros beetles that develop in poor conditions or with inadequate substrate produce very small horns, sometimes barely visible. These are called "minor males," and they can look confusingly similar to females if you don't know what to look for. Check the head shape and look for even a small nub where the horn would be.
Flower beetles
With flower beetles (Pachnoda, Mecynorrhina, Eudicella), the differences are more subtle but still visible.
In Mecynorrhina species, males have a pronounced horn or projection on the head. Mecynorrhina torquata males have a distinct upward-pointing cephalic horn that females completely lack. Eudicella males also have forked head projections that are absent in females.
For Pachnoda marginata (sun beetles), it's trickier. There's no horn. The most reliable method is checking the underside of the abdomen. Males have a slight groove or concavity on the last abdominal sternite, while females are more convex. It's subtle, and honestly easier to see when you've got both sexes side by side for comparison. Males also tend to be slightly smaller, though there's enough overlap in size that you can't rely on that alone.
Sexing larvae
This is where it gets fiddly. Beetle larvae can be sexed, but the method depends on the species and the larval stage. It's typically only reliable at L3 (the final instar), when the larvae are large enough to examine properly.
For many scarab beetle larvae (rhinoceros beetles, flower beetles), the standard method is looking at the ventral surface of the last few abdominal segments. Males of many species have a small marking called the Herold's organ (sometimes called the Herold organ or male spot), visible as a faint darker patch or pair of small dots on the ventral side of the penultimate abdominal segment. You need good lighting and a calm larva to spot it.
With Dynastes larvae, experienced breeders can sex them fairly reliably at late L3 by weight as well. Males tend to be significantly heavier than females at the same stage, because they're building the body mass needed to develop horns during pupation. But weight alone isn't definitive.
If you're new to this, don't stress about sexing larvae. You'll know soon enough once they eclose. Disturbing larvae to examine them carries risk, especially if they're close to pupation.
Stag beetles
Stag beetles (family Lucanidae) are another group where males are obvious. Males have enlarged mandibles that can be as long as the rest of their body in some species. Females have much smaller, functional mandibles. In species like Lucanus cervus (the European stag beetle) or Dorcus titanus, the size difference in the jaws is impossible to miss.
Like rhinoceros beetles, stag beetles can produce minor males with reduced mandibles. These are still distinguishable from females by head width and mandible shape, even when the mandibles aren't dramatically enlarged.
Why it matters for breeding
Beyond the obvious need for both sexes, knowing who's who lets you manage your group better. If you've got six beetles and five are males, you're not going to get many eggs. Males of some species also fight, and housing too many together in a confined space leads to damage and stress. Trypoxylus dichotomus males in particular will wrestle aggressively, using their horns to flip rivals off branches.
Female beetles generally live longer than males in many species, because they need time to lay eggs after mating. A male that dies a few weeks after eclosion has done his job. A female that dies before laying has not. If your females are dying early, look at your conditions, particularly temperature and food quality.
When in doubt
Some beetle species are genuinely difficult to sex without experience. Darkling beetles (Tenebrionidae), for instance, often show minimal external sexual dimorphism as adults. If you're struggling and it matters to you, ask in one of the beetle-keeping forums or groups. A clear photo of the beetle's underside usually gets you an answer within a day. The hobby community is generally good about helping with identification.