Set a voltmeter to the x1 setting.
Furnace blower capacitor test.
Remove the blower housing inside the furnace to access the capacitor.
Remove the wires from the terminal and discharge the capacitor using the instructions included with your furnace.
The capacitor will still hold a charge even without power.
Shut off the power to the furnace at the fuse.
A furnace usually uses a run capacitor to maintain a relatively constant voltage supply to the blower motor.
If the system is not running then a bench test will be the best bet.
Do not do an under load test on blower capacitors due to the risk of the meter leads around a spinning blower wheel.
If the system is currently running then an under load test may be best.