Ohlins state steering dampers should be rebuilt every 3,000, shocks closer to 6000 miles but depending on use it can be longer or short than that.

If the damper is off the bike then about all you can do is check for leaks and stroke the damper its full lenth to feel for any air. If the damper is in need of a rebuild you can normaly feel an area in the stoke where the damper lacks damping by moving it back and forth. Most of this though requires romoval of the spring.

They are quite easy to service and your local yamaha dealer should be able to tell you where the nearest ohlins service center is.
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If I have to take the carbs off once more...