The wing has no idea where the ground is (below the aeroplane, above it, in front of it, behind it, or to either side). All it understands is angle of attack (and structural loads, of course). Exceed the critical angle, and it will stall.
Many wings are optimised for erect flight so have a more cambered upper surface than lower, so the critcal angle for stalling the lower surface will be less than for the upper surface. A symmetrical wing will stall just the same either way.