How do you check the pre-charge of an accumulator with an "exclusive" gas port, when your charging kit does not have the right accessory?
Well, it is not difficult at all, because you still have access to the "wet" side! If you install a pressure gauge in the accumulator line and pressurize it to the point when some oil "gets inside", and then vent the pressure slowly while constantly monitoring the gauge - when the bladder expands fully and the oil flow from the accumulator stops there will be an abrupt drop of the pressure reading (easily seen on an analog gauge). The "pressure drop point" is the pre-charge pressure. Piece of cake!
Often it is possible to perform this check without removing the accumulator from its place since it can be pressurized by the system pressure, and then the manual safety discharge valve (if equipped with one, of course) can be conveniently used for the "slow venting". If you are lucky, the accumulator line may even have a test port already installed. If it does, pinch yourself to make sure you are not dreaming. This is my way of saying that hydraulic installations must have pressure test ports installed during the construction phase in all key places. Test couplings are NOT a waste of money, people!
This method has the advantage of being non-invasive - i.e. you don't connect to the gas and therefore waste none of it. This can make a difference for very small accumulators, which lose a relatively large part of pre-charge every time a direct gas pressure measurement is performed.
Although this method doesn't provide the reading accuracy of a direct gas pressure measurement, it is still accurate enough 99 times of 100.
Now for the visual demonstration...
A long-long time ago, I made a video of how such a pressure check can be performed, and it is still up on YouTube. It explains the concept pretty well, but you can tell that it was shot with a brick. I am still placing a link to it here, mostly for historical purposes, because it serves as a good record of how prehistoric phone cameras used to work. If you ever choose to watch it - be warned - the resolution is at astonishing 240p! I'll re-shoot it and update the link when I have a chance. Don't say I didn't warn you!