Seeing as noone has answered I'll have a go. Do bear in mind I know next to nothing about kegging (although I do know a wee bit about mini kegging) or gas.
S0....Would attaching some sort of pressure guage work? (I'm basing this guess on the pressure guages you can get for bike tyres to find out what the psi is )
Assuming you bought your S30 to pressurise something why not connect it up? If you twist the cylinder to open the cylinder valve NRV nozzle you will hear CO2 passing. But if the cylinder is completely empty the nozzle thingy will be loose.