The anal fin settles a young guppy's sex
The small fin on a guppy's underside, just in front of the tail, is the key to sexing youngsters. On a female it stays a neat triangular fan that opens and closes as she swims. On a male it transforms as he matures, narrowing and rolling into a pointed rod, the gonopodium, which he can swing forward when mating. Very young fry all look female at first because that transformation has not begun, so check growing guppies repeatedly over their early weeks; the day one youngster's fan starts looking like a stick, you have found a male.
Colour and size back up the fin
A young male guppy advertises himself in colour long before he is fully grown. Even in a plain strain, males develop brighter body colour, bolder tail patterns, and flowing finnage while staying the smaller, slimmer sex. Females grow larger and deeper-bodied, usually in quieter silver-grey tones with colour confined mostly to the tail. In fancy strains both sexes can be colourful, which is exactly why the anal fin outranks appearance: a vivid young guppy with a fan fin is female, however showy she looks, and a drab one with a gonopodium is male.
The gravid spot and what it does and does not mean
Many female guppies show a dark patch, the gravid spot, on the rear of the belly just above the anal fin. It deepens when a female is carrying developing fry, but it can be visible in young females before any pregnancy and faint or absent in some colour varieties, including pale and albino types. Treat the spot as a supporting female clue, never the deciding one, and do not read a dark spot on a young female as proof she is about to give birth. The fan-shaped fin beneath it is the confirmation that matters.
When to separate youngsters, and why it matters
Sexing young guppies early is practical, not just curious. Males and females mature fast, and a mixed group left together will produce fry sooner than most new owners expect, filling a small tank within a couple of generations. Keepers who want to control breeding separate the sexes as soon as gonopodiums appear, while those happy to raise fry can leave a community together with plenty of plant cover. Either way, the fin check has to happen on schedule, because a fortnight's delay in a fast-growing batch of guppies can be the difference between sorting and surprises. Keep a pencil tally on the tank stand if it helps: numbers of sorted males and females, updated at each check, stop the same fish being judged twice while another escapes notice entirely.
Keeping track of a fast-changing group
A guppy group changes weekly when it is young, and memory blurs which fish showed what. Note the date each youngster's fin changed, which tank each sex moved to, and any female who arrived already carrying fry from a mixed shop tank. Writing those details against each fish in a KinsLe Pet Passport gives every guppy a small portable record, so its sex, strain, and history travel with it if the group is split, sold on, or moved to a new aquarium.