Is that the old type E France plug?
More basically,initially,
I would get a 100 kOhm 1 ~ 5 Watt Resistor:
(220 ^2) /100000 =0.48 Watt
Connect across the pins of an old , spare plug.
Now you can check with a multimeter, which side is lowest voltage and which side is "hot" to ground.
That is assuming you know how to do it safely, and there is a ground like embedded water pipe, steam heater pipe etc.
With old Test Equipment, Radios , other appliances,
the first thing is to check that the On/Off switch and appliance fuse (if fitted) is on the "hot" side.
With newer equipment I presume somebody already connected the outdated plug? So you can open it and check tightness and no splayed strands. And how is the ground wire terminated.
The ground test will also tell if there is residual voltage on the side that is supposed to be at ground potential.