Paul,
I check all of the following at least annually.
Internal leaks:
Internal leaks are checked by removing the nose gear fork. The hollow interior should remain dry. If the tiny O-rings of the guide key begin leaking, hydraulic fluid will start filling the hollow interior of the tube. There may be no exterior signs of a problem. The tube will just keep filling until the shimmy damper starts pulling in air.
Fluid Level:
This can be check by depressurizing and fully collapsing the strut. Remove the valve and using a syringe check to see how much fluid is needed to bring the level back up the valve opening. Fluid will be lost in normal operation, even if it is only a very small amount. Since shimmy is so catastrophic it is important to know the rate at which fluid is leaving the system. If the level ever gets to low the damper will not work. The idea is to catch leaks and low fluid levels before they can produce a shimmy event. It is sort of like checking engine oil. We are all familiar with our engine's oil burn rate, but we still check the oil level during pre-flight. We want to catch any sudden change.
Damping Force:
This is a little more challenging if the self centering feature is installed. It requires two flat and smooth metal plates with grease between them. With the tow bar attached the strut should be rotated through its full range of travel. I am not aware of any values for damping ever having been published, but experience with newly overhauled struts will give an indication of the proper resistance. It is very difficult to turn the strut by hand just grabbing onto the wheel. In addition to overall resistance, it is important to feel for any dead spots. This indicates either worn parts or air in the damper. Newly overhauled units need to be cycled back and forth a lot to remove all the air - on the order of 20-30 cycles.
Even with all the oil removed you wouldn't see much of any change in strut extension. Temperature has a much greater effect on strut extension than fluid level.
hope this helps,
Chris
Chris Zavatson
N91CZ
360std