Usually the slave will fail slowly. You will start to notice that it slowly lets out even when fully depressed while waiting for a traffic light in gear, or whatever.
Intermittent or sudden failures are usually the master.
I assume you have already checked the fluid level.
Sounds improbable, but a friend had an intermmitent clutch failure caused by low fluid level on a 124, because it has a single reservoir for brake and clutch, and the clutch fluid intake is higher than the brake intake (which is at the very bottom). It would fail on a certain grade (facing uphill?), but worked otherwise!
1986 300E 5-Speed 240k mi.