No visual way of checking. Inside the sphere is a rubber diaphragm. Air (nitrogen gas?) is trapped above the diaphragm under pressure and the hydraulic acts on the underside of the diaphragm. This aborbs the fluid displaced when the suspension moves and the damper telescopes.
Most cars with conventional suspension have this diaphragm/air assembly inside the shock (hence the term 'gas filled') but for self levelling to work it needs to vary the volume of fluid in the system.
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)