Set up your ladder. Make sure that your ladder is stable and that you can easily reach the ceiling.
Climb the ladder with your hammer and nails.
Lightly tap different spots of the ceiling within your reach.
Listen closely to the sound of each tap. Places in your ceiling that are not under a beam will make hollow sounds when you tap them. Places that are under a beam will make a more solid, thudding sound.
Verify the beam's location by driving a nail a short distance into the ceiling. If that is the location of the beam, you will feel resistance from the beam after the nail penetrates the surface of the ceiling. If not, the nail will drive in cleanly and easily.
Drive in additional nails to determine the width and center of the beam.
Remove all the nails that you drove into your ceiling.