How do I test and maintain the expansion tank on my boiler?

I don’t know what maintenance or adjustments are possible on an expansion tank. I understand from the answer to this question that it needs to be "pre charged" to accommodate the range of pressures that will be encountered during use. However once it is connected to the system and under pressure from the water side, it seems to me it is impossible to test or perform maintenance on the air side.

• Is there any way to test or make corrections to the air pressure while it is connected to the system?
• My expansion tank, pictured, is meant to be floor standing with the air valve on top so the advice often seen to test its integrity by seeing if water comes out the air valve cannot be done on my tank, right?
• If the system is full of water and the pressures when cold and hot are as expected and the relief valve isn’t popping, can I safely conclude that the expansion tank is doing its job?
• Should I, (perhaps the next time I have to empty the system), install a ball valve and pressure relief on the water side of the expansion tank so that I can remove it, test its integrity, and test its precharge pressure? Is that something I ought to do periodically?
• If I do disconnect and reconnect the expansion tank, how do I bleed air out of its water side? It seems to me that is impossible, but also unnecessary because a little air there will take on the system water pressure and so the expansion tank will still work as it should.