Hi lahoanghai,
Why we have to verify spoiler avail? (operate spoiler to confirm system pressure avail) , instead of other flight control surface.
See QRH, Operational Data, Flight Controls Architecture.
The individual spoilers are powered by a single HYD system, so if a HYD SYS pressure fails, the spoiler won't move.
The other flight control surfaces have at least 2 HYD Sytems powering them, so if only One HYD SYS fails, the surface will still move.