You know when it starts slipping at full throttle in the higher gears.
If you are not drag racing it or otherwise abusing it, it should be fine. As it wears the engagement area will change slightly.
If you want to check it out, pull it apart and measure everything per the service manual. Most of the time you can freshen a worn clutch by and swapping for the thin steel plates for the thick ones along with a fresh set of springs.