The tail is the health gauge
With a leopard gecko, one look at the tail tells you how the last few months have gone. The tail is this species's fat store, and a well-fed, unstressed gecko carries it thick and smooth, nearly as wide as the neck at its base. A tail that is thin, wrinkled, or reed-like belongs to a gecko that has not been eating, has been outcompeted by cage-mates, or is carrying a parasite load, and a beginner should pass it over. Also check the tail's outline for kinks or a regenerated tip, which is shorter and blunter than the original; regeneration is not a health problem in itself, but it tells you the gecko has dropped its tail under stress at some point, which is worth knowing.
Toes, eyes and skin up close
Ask to see the gecko in your hands or in a clear tub under good light. Leopard geckos cannot climb glass and shed in one pale sheet, and the trouble spots are the toes: look for grey-white rings of retained shed constricting them, and count the toes, since shed left on for weeks quietly costs geckos their toe tips. Eyes should be clear, open, and free of crust or retained skin, and the gecko should blink and lick its eyes clean. Run your eye over the skin for tiny dark or reddish dots, especially around the eyes and armpits, which can be mites, and check the vent is clean and flat rather than swollen or crusted. The belly should feel firm and smooth; a hard lump or a large dark patch visible through the pale underside is a reason to choose a different animal.
Behavior at shop-closing time
Leopard geckos are dusk animals, so a gecko judged at noon is a gecko half asleep. Visit in the late afternoon or evening if you can, when a healthy gecko comes out of its hide, walks the enclosure with its belly held clear of the ground, and shows interest in the world. Healthy movement is steady and deliberate, without trembling, stargazing head tilts, or dragging limbs. Ask what insects it has been fed, dusted with what, and when it last shed and ate; a good shop tracks this per animal. A gecko that tongue-flicks, explores an offered hand, and settles without frantic struggling is showing the calm temperament this species is loved for.
Questions to ask before you pay
Finish with the record-keeping questions. How old is the gecko, and is it a proven feeder on the insects you plan to offer? What temperatures has its enclosure been running, so you can match them at home for the first weeks? Has it been housed alone, as adults should be? A shop that answers directly and shows you clean, singly housed geckos on solid substrate with proper hides is a shop whose animals you can trust. Take the answers home in writing if they will give them, because those first notes become the opening page of the gecko's health record: keeping them in KinsLe's Pet Passport means the first vet visit starts from real history rather than your memory of a shop conversation.