The quick answer for a young Chinese hamster
A young Chinese hamster is most reliably sexed by spacing underneath, not by colour, size, or temperament. In a male Chinese hamster the distance between the anus and the genital opening is noticeably wider, and his rear profile looks longer and more tapered. In a female Chinese hamster those two openings sit close together, her underside looks smoother between them, and her back end reads rounder overall. Neither clue alone settles a very young pup, so read both together before you label anyone in the litter.
Why Chinese hamsters fool people at first glance
Chinese hamsters stay small and slim even as adults, so the obvious bulk people expect in a male hamster never really appears. A young male Chinese hamster can look almost identical to his sisters from above, because his testicles, while proportionally prominent for this species once he matures a little, may not be obvious in a squirming juvenile held at the wrong angle. Colour patterns mislead too: a bolder stripe or a slightly larger pup tells you nothing about sex. That is why a top-down guess in the enclosure so often gets reversed a week later, and why the underside spacing check, done calmly, is the method worth trusting.
A calm, low check step by step
For a young Chinese hamster, work over a towel on a table or inside a shallow box so a startle cannot become a fall. Scoop the pup up in cupped hands rather than grabbing from above, let him settle for a moment, then gently tip him so you can see the underside without pinning him flat or stretching him out. Look first at the gap between the two openings: a clear stretch of bare skin points to male, openings almost touching points to female. Then glance at the rear outline from the side: tapered and extended suggests male, neat and rounded suggests female. Keep the whole check brief, return him to familiar bedding straight away, and wash your hands before handling the next pup so scent and stress do not carry across the litter.
Signs that should make you pause and recheck
A young Chinese hamster should be rechecked rather than labelled if the spacing looks intermediate, the pup was too tense to view clearly, or siblings you compared against gave contradictory impressions. Youngsters change fast, and a male who looked ambiguous can look unmistakable once his rear elongates a little more. Comparing littermates side by side, in the same position and light, often resolves the doubt better than staring at one pup alone. If you are separating youngsters into new homes or single housing, an uncertain call matters, because Chinese hamsters are generally kept alone as adults and a mis-sexed pair can produce an unexpected litter. Ask the breeder, rescue, or a veterinarian familiar with small mammals to confirm any pup you still cannot read after a second calm attempt.
Recording the result so it sticks
Write down the sex of each young Chinese hamster the moment you confirm it, alongside a distinguishing note such as a stripe detail or temporary safe marking agreed with the breeder, because memory blurs quickly when several near-identical pups are involved. A note made on the day also helps the next owner, who deserves a confident answer rather than a guess passed along. Photographing the underside check, with help holding the pup, can give you a reference to compare against later without handling him again purely to re-verify. Keeping that confirmed detail on file with KinsLe through Pet Pass means your Chinese hamster's sex lives on his own verified ID record, ready to show accurately whenever his identity needs presenting.