How do I know if my kid has worms?

In the UK and US, childhood worms almost always means threadworms, and the NHS description is distinctive: extreme itching around the anus, particularly at night, and small white worms in poo that look like pieces of thread, sometimes visible around the child's bottom or on their night clothes. The itch happens because the female worm comes out to lay eggs at night, which is also why kids with threadworms sleep badly and scratch. Confirming it is easy: check the child's bottom with a torch a couple of hours after bedtime, or look at the morning poo. Treating it is also easy: the NHS says threadworms are common and can be treated with medicine from a pharmacy, no GP visit needed for the standard case, though everyone in the household usually needs treating at once and the medicine may need repeating after two weeks. The hygiene fortnight matters as much as the medicine: wash hands after the toilet and before food, keep fingernails short, wash sleepwear, bed linen, and towels hot, and damp-dust the bedroom, because eggs survive on surfaces and reinfection is the norm when hygiene slips. See a GP instead for a large worm in poo, a worm-shaped itchy rash on the skin, stomach upset lasting over two weeks, or unexplained weight loss, which suggest something other than threadworm.