As Mat says, you will only know when the box is off unfortunately, but if it has more than 2 teeth's worth of play then it's time to replace. My mates was about fooked when he checked his and like a lot of people couldn't really have the car in bits and then have to wait for a new flywheel to be delivered, so ordered both before hand.. His had about 20k on it too, but that doesn't mean your's will necessarily be fooked.. Just that it may..
His was slipping in most gears, but not ALL the time.
Not really what you want to be hearing I know mate, but going off other people having similar issues, it may cost you a bit more than you had hoped..