King Air's use a cabin altitude sensing switch for automatically deploying the passenger masks when cabin altitude exceeds approximately 12,000 feet. (I did not investigate it, but I believe another switch is used for crew warning.)
The switch is located in the forward cabin, right side-wall, just forward of the emergency exit.
When excessive cabin altitude is sensed, the switch closes and provides 28VDC to the control solenoid (horizontal silver barrel) of the cabin oxygen valve. A crew activated manual cable can be seen attached to the aft side of the valve.
Several methods of testing are mentioned in the Maintenance Manual. One of which is testing the switch while still connected to the aircraft, which seemed difficult to almost impossible. Another method allowed for the switch to be removed and tested separately.
A test chamber is required. The switch is attached to some internal wires and placed inside the chamber which is then sealed. A vacuum can be applied via a feed-though AN fitting. With the proper altitude simulated, the switch will close. Continuity can be read with a meter attached to the chamber's test wires.
Actual mask deployment can be verified by jumping the aircraft side switch wiring. The valve solenoid can be clearly heard activating. (I found a broken suppression diode
on this particular aircraft.