You pump it up then check for leaks around the hose connections, under the hydro `boot' rubbers and particularly at the 2 fill valves in the back. These are just normal Schrader tyre valves, change if they leak.
If it's always had pressure in they don't give much trouble- mine are now 38 years old and counting...
If you buy the car I would dump the old scungy fluid by letting it out, and refill each side by pumping up with fresh. As long as you don't dismantle anything this will NOT let air in.