[Amps] Testing / Using Spare Tube

[Paul Jansen]

I have an Alpha 78 that I have been using for 6 months or so that came with
a spare tube.  
The amp is working fine with full (or very close to full) output with the 3
existing - possibly original - tubes.
Supposedly, the spare has never been used - "only tested to be sure it was
still good"

So, what is the recommended procedure to test this spare tube?
Should I be trying to rotate this spare (supposedly "new") tube into service
- and if so what precautions need to be followed?

While I do have a fair background in electronics, having been involved in
service and repair for years - my experience does not extend to the area of
RF amplifiers and tubes.

Your advice would be greatly appreciated.

(I did try to search the archives first - but if the answer was out there
somewhere it managed to elude me - sorry...)

Thanks,

Paul / NO9C