Verify the claim under the fish's scientific name, not its shop label. A claim made beside a tank of juveniles is a claim about an animal you cannot see, so your job is to find reliable evidence about the adult before the fish is in a bag on the way home. That evidence exists, and it takes only a few minutes to gather if you know which name to search.
Get the Scientific Name First
Ask the shop for the scientific name and write it down exactly, including the spelling. Common names are reused freely. One name can cover several species of different adult sizes, and a label can also be shortened or invented for the tank. If the shop cannot supply a scientific name, treat every other claim from that tank with caution, because the shop itself may not know which species it is selling. A photograph of the fish beside its label helps you confirm the identification later if the name and the fish in the bag ever seem to disagree.
Compare Two Independent Sources
Check the scientific name in two sources that do not simply copy each other, such as a specialist species database and an experienced aquatic society or retailer guide. Compare the adult size each one gives. Agreement between two sources is reassuring, while a wide gap between them is information too, because it tells you that reported sizes vary and that your tank plan needs the cautious end. Note whether each source describes a typical home-tank size or a maximum recorded size. Those are different figures, and a shop quoting the typical figure as a guarantee is smoothing over the variation that your tank will have to absorb.
Read the Size Figure Carefully
Check what the quoted length actually measures. Some sources give standard length, which runs to the base of the tail, while others give total length including the tail fin, so the same fish can carry two honest numbers that differ noticeably. Sex can matter as well, since in some species one sex grows larger and a quoted figure may describe the smaller one. Look for photographs of mature specimens in home tanks rather than only illustrations or juvenile shop photographs. A mature fish photographed beside recognisable tank equipment gives your eye a scale that a bare number cannot.
Ask the Shop to Show You an Adult
A shop that regularly sells a species often keeps an adult in a display tank, a breeding setup, or a staff member's home tank, and asking to see one is a fair request. If no adult exists anywhere the shop can point to, ask how the stays-small claim is known. Be especially careful with any fish sold tiny, cheaply, and in large numbers, because that profile fits species that grow very large and are produced in huge quantities as juveniles. None of this makes the shop dishonest. It makes the claim untested, and untested claims should be priced and planned as unproven.
Plan the Tank Around the Largest Figure
Where your sources disagree, stock and equip for the largest adult size you found. That decision protects the fish if it grows to the top of its range, and it protects your other fish from the crowding that follows an undersized plan. If the largest figure does not fit the tank you own, the correct outcome is a different fish, however appealing the juvenile looked. Record what you verified before you buy, so the decision rests on your notes rather than on a conversation you half remember. KinsLe's Pet Passport is a good home for that record, because the scientific name, the adult size you planned around, and the purchase date can sit together with the fish's feeding and water notes.