Was/Is it connected to a capacitor?
Is the capacitor a polarized electrolytic of some "oil filled" non-polarized thing?
You might determine the polarity from any possible capacitors or other circuitry is is/was connected to.
Generally, if it is something I am pulling apart, but wish to ultimately restore function to, I will draw the circuit first so I can see what went through the head of the designer...often it is not what you would traditionally expect.
Example, "Gigastrobe" units...don't use capacitors, least not here in "240v, 50Hz land, they simply have a large diode and a low value inductor and the xenon tube all in series right across the mains. When the tube is fired, it conducts for one half cycle and extinguishes at the next zero-cross. (Which is pretty rough on the building's switchboard and distribution transformer). At very large discharge currents xenon discharges exhibit a POSITIVE thermal resistance coefficient, so there is something to prevent a catastrophic failure. (In Russia they had 100Kw + xenon tubes that connected directly across 415v phases, these were about 8' long or more).
So, can you elucidate more about this inacessable xenon tube.
Why is is so inaccessable?
Is it in a piece of existing gear whose function must be restored?
Is it in a new design?
Is it being re-purposed from a failed piece of gear into a new design?
Is it in a capacitor-discharge type flash/strobe circuit?
Is it in a non-capacitor circuit like the one described above?