What's the maximum pressure one airtank can withstand and how to tell its limit?
You will not destroy the air tank using standard LEGO components. I don't know what it's structural limits are, but I can tell you that the weakest link in the pneumatic system in my experience is the connection between the rubber pneumatic tubes and plastic inlets. Under high pressure, the tubes will simply pop off before any of the components will actually break.

I believe that the most pressure you are going to be able to achieve using standard LEGO components is around 40psi. All of the components can easily handle this pressure.

If you'd like to do some exploration of this on your own, you might want to get your hands on the pneumatic pressure gauge that has been available in the LEGO Education line:



If you really want to know the maximum pressure limit of the tank itself, you'd probably need to somehow attach it to real compressor and wait for it to break. I like my pneumatic components too much to do this myself. :)