To locate a wooden joist (or beam/stud), gently tap the plasterboard (or drywall) surface. A hollow sound indicates there is no joist, keep tapping until a solid sound indicates the joist location. To save your knuckles or fingers, you can use the end of a screwdriver or something like a wooden spoon.

You can also double-check using a magnet to locate the line of the existing drywall screws that secure your drywall into the joist. Simply slide the magnet up and down the drywall surface until you feel it being attracted to an existing screw. Do the same vertically up and down the wall to locate the other screws that attach the drywall to the joist.

Also, you can check by creating a test pilot hole with small drill-bit (2-3mm diameter) where the pilot drill-bit should butt against the joist rather than just pushing straight through the surface of your drywall. 

Another option is to use a wall scanner/stud finder. We would always recommend using a decent quality brand (such as Bosch) to locate studs and any services in your walls.