How do you find the joist in a wall?

To find a joist in a wall, you can use a stud finder, which detects changes in density behind the wall surface. If you don't have one, you can knock on the wall; a solid sound indicates a joist, while a hollow sound suggests a space between them. Joists are typically spaced 16 or 24 inches apart, so measure from a known point to locate them accurately. Additionally, you can look for visible signs, such as nails or screws, on the wall surface.