What actually works to find a stud without a stud finder?

a strong magnet. slide it slowly over the wall and it'll grab the screws or nails holding the plasterboard to the stud. once you've got one, the others are normally at regular spacing either side. knocking works too, hollow sound between studs, duller and more solid over one. light switches and sockets are usually fixed to the side of a stud, so that's a good place to start. before drilling anything big, confirm with a thin nail somewhere the hole will be hidden.