I have recently acquired one of these GEC barretter lamps in a trade with Lucas (LAllenLighting) and I was wondering if anyone knows of a safe way of testing it..? It obviously has to be wired in series with something, what could I use and what voltage would be safe..?

It is supposed to regulate a load current when connected in series with other resistive loads (like vacuum tube filaments,...) to a constant voltage supply.
So connecting to a voltage source within the rating limits should be OK.
But question is, whether you will be able to find the voltage range rating info.

Be careful, these use to be designed to barely glow dark, to stay within the gas convection being the dominant mode of filament cooling (then it uses to yield a really constant current within very broad voltage drop range), so may use filament material not so resilient against so high temperatures as e.g. tungsten. So judging the maximum voltage according to the color is not that reliable.
But if the filament is really tungsten, then orange glow for a short time would be fine (wen trying on something like a variac).

Thanks, yes I did run it up on the Variac and it started to glow dimly at around 40V.