It's 14,000 feet to set off masks automatically.
I don't know what your paperwork reads, but if I wanted to test the altitude switch..... I'd pull relay R322 and look for a ground to show up on "X2".
If you test with everything hooked up, the masks will drop. "Pass Oxy Left" and "Pass Oxy Right" breakers could be pulled to test the system without actually dropping the masks.