Since I owned my car I had rear suspension and ride issues. some times the car ride great and other time is flat out horrible. I am finally close to giving up and buying new shocks and new cells. Before I spend the money I was wondering if someone can provide me with a testing process to determine the problem before I can invest in new($$) shocks. I searched through the archives but could not find a clear answer.
My questions are:

How would you test if the pump is in working order?
How would you test if the air cell are in working order?
How would you test if the control valve are in working order?
How would you test if the air cell are in working order?

The air cell has 30,000 miles on them. The car has 143,000 miles. The ride height is fine but the tires sag a little. If you bounce the back it is soft but not too much movement.

Any help will be great.