When doing a pre shift inspection there are 2 different requirements to checking the machines hydraulic systems. The first is to check the hydraulic fluid levels in the reservoir, this is done by looking a fluid level window or dipstick.
The second requirement is to physically observe each hydraulic cylinder on the machine. This is done by fully extending them and viewing the where the shaft meets the cylinder. If any fluid is found on this shaft (or the seal is visually broken) that cylinder needs to be serviced.