Bench or under load test.
Furnace blower capacitor test.
If the system is currently running then an under load test may be best.
If the system is not running then a bench test will be the best bet.
The capacitor will still hold a charge even without power.
Shut off the power to the furnace at the fuse.
Set a voltmeter to the x1 setting.
Do not do an under load test on blower capacitors due to the risk of the meter leads around a spinning blower wheel.
Decide if under load or bench testing is best.
A furnace usually uses a run capacitor to maintain a relatively constant voltage supply to the blower motor.