Feeler gauges are fine for scheduled checks but if you want early warning, put a couple of eddy current proximity sensors on the slide. We mounted two — one measuring X-axis play, one measuring Y-axis. Feed them into the press controller and set alarm thresholds. You'll see gib wear developing weeks before it shows up in part quality.
The other thing — check your centralized lube system is actually delivering oil to the gibs. We had a press where the gib lube line had a kink from a maintenance guy stepping on it. Gibs wore out in 6 months instead of the usual 24. Now we put flow indicators (those little spinning wheel things) on every gib lube line so you can visually confirm flow during operation.
On the composite gib point above — agreed, but make sure your press builder approves the switch. Some frames are designed with specific gib materials in mind and the thermal expansion coefficients matter. We tried aftermarket composite gibs on a Komatsu and they worked great in summer but gapped in winter when the shop dropped below 10C.