Unplug the appliance and disassemble enough to access the switch. Disconnect one lead from the switch.

Set the multimeter on RX1 scale. Clip probes to the switch terminals or leads. Turn the switch on (for multi-speed switches, press one switch at a time and note each reading). Zero ohms means the switch is OK. High or fluctuating ohms means the switch is broken or dirty.

Clean any switch contacts with a small file or with electrical contact cleaner. Contacts should make firm connection when the switch is on. If they don't, replace the switch rather than repair it.

Use electrical contact cleaner to clean less accessible switch contacts. In order to work cleaner into the switch, operate the control buttons as you spray the cleaner into apertures. Don't overspray.