If I remember right I just checked with a plasma ball if they gassed out. If the gas filling is OK you will induce gas discharges when holding them near such a device while dead ones don't show the typical discharge color. The currents are low enough.
This is not a test for functionality, but it gives a first impression if the tube is dead to to vacuum failure...
I just checked a bunch of similar tubes with my plasma globe. The neon discharge will start when the tube I hold at the wires is some cm away from the glas bowl. For my current tube delivery the result was not too promising, four of the delivered 20 tubes were dead (gas leak).
As said, I previously pre-tested my set of ITS-1A before soldering them into Jürgen Grau's Thyrathron kit. It didn't fry my tubes and was easy enough to handle.
Here's my latest new clock I recently acquired. I'm hoping it will be all it can be. I wanted to build one but the PCB's aren't available anymore I don't think. I think I may have missed the boat on building one unless I got some more PCB made: