The black wedge is a harlequin's name badge and clue
Every harlequin rasbora carries a dark triangular wedge on its rear flank, and that marking holds the first sexing clue. In a maturing male the wedge tends to look bolder and more complete, its lower front corner extending and rounding down toward the belly line. In a female the wedge's front edge usually runs straighter, giving the triangle a cleaner, more upright boundary. The difference is subtle enough that lighting and mood change it, so use the wedge as one vote among several, and only on fish old enough for their adult pattern to have settled fully.
Slim males, rounder females in the same shoal
Body shape carries the second vote in a harlequin rasbora group. Males stay slimmer and more streamlined, and their copper-pink colouring often deepens, especially when the shoal is excited at feeding time. Females grow a little larger overall and fill out through the belly, so from above or head-on a female looks gently rounded where a male looks straight-sided. In a young shoal these contrasts are faint because everyone is still growing, but they sharpen as the group matures, which is why comparing tank mates of identical age and diet beats judging any fish in isolation.
Why young rasboras defeat one-off judgements
A young harlequin rasbora is a small, still-colouring fish whose wedge has not finished darkening and whose belly has not yet taken an adult line. Shop lighting flattens the copper tones that separate bright males from paler females, and a fish pale from the stress of bagging and travel shows none of its best colour for days. Sexing on purchase day is therefore mostly wishful. Feeding time offers the fairest comparison of all, because the whole shoal rises together into the light and holds near the surface for a moment, bellies and wedges on show in the same glance. Let the shoal settle in your water for several weeks, feed them well, and then read the group in calm morning light, when rasboras show their honest colours and natural shapes.
Behaviour at spawning time confirms the rest
If your harlequin rasboras ever approach breeding condition, behaviour finishes the identification. Males display to females with quivering fins and heightened colour, competing to lead a female toward broad plant leaves. Egg-laying harlequins famously turn upside down beneath a leaf to attach their eggs, and the rounder fish doing the laying is female beyond argument. Most community shoals never spawn, so treat this as a welcome confirmation rather than a method to rely on. Until then, wedge, waistline, and colour together give you a reasonable, honestly hedged answer.
Living with a best-effort answer
For a community shoal, an approximate sex ratio harms nothing, since harlequin rasboras school peacefully in almost any mix. The answer matters mainly to keepers hoping to breed them or balance a very small group, and both can work from careful comparison rather than certainty. Keep notes on which individuals look male or female and revisit them as the fish mature, because a young rasbora's story often changes. Sharing that revisiting as a light recurring duty in KinsLe Care Teams, with each observer's note timestamped, turns scattered glances at the shoal into a record you can trust.