Remove the blower housing inside the furnace to access the capacitor.
Furnace blower capacitor test.
A furnace usually uses a run capacitor to maintain a relatively constant voltage supply to the blower motor.
Remove the wires from the terminal and discharge the capacitor using the instructions included with your furnace.
Do not do an under load test on blower capacitors due to the risk of the meter leads around a spinning blower wheel.
Bench or under load test.
If the system is currently running then an under load test may be best.
Set a voltmeter to the x1 setting.
Decide if under load or bench testing is best.
Shut off the power to the furnace at the fuse.