Get on the ladder and have someone else hold it if it feels unstable. Knock on the ceiling with your fist until you find a stud -- the ceiling will make a hollow sound where there is no stud, and a lower, duller sound where there is one. Determine which direction it goes in by repeating the knocking process in different directions to see where the stud ends. Mark it with a pencil.
Turn off the power to the area in which you are searching for wires. Since ceiling wires can run to several different rooms, the safest option is to turn off the power for your entire house.
Press an electronic stud finder against the ceiling where you marked your stud.