Run the engine at idle and make sure the fan comes on before the temperature gauge goes past the halfway point (this assumes that you have the lower temperature fan switch, which you should have). If the fan does not come on, see cooling fan troubleshooting notes in the troubleshooting section.
Pull the fuse for the cooling fan fan low speed (relay/fuse box, right front of the engine compartment as you stand facing the car). With the engine at idle, make sure the fan comes on at high speed before the temperature gauge goes past the three-quarter point. If the fan does not come on by then, replace the low speed fuse and drive the car between 30 and 40 mph until the temp gauge returns to well below the three-quarter point. Then see the troubleshooting section. Again, this assumes that you have the lower temperature fan switch.