They are cheap so I wouldn't take a chance. If you can hear liquid when you compress the shock you are hearing bubbles form air that has replaced a full charge.
A new one will be significantly stiffer than old, but unless you have the new one the comparison will be hard to describe.
BTW, I personally can only slightly tell the difference between no shock and a good shock, if everything else is tight and in balance.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician