(1)
You can find the GND (center POS/NEG) by probing with a multimeter. Probe around the capacitors on the PCB, as they all have a negative pin on the GND side and most likely no direct connection to the POS input.
You could also identify a component pin directly soldered to the GND-fill on the PCB visually.
(2)
By connecting a variable voltage-source to the POS pin and ramping it slowely from 1V8/3V3/5V0 until the circuit starts operating. I would personally guess either 3V3 or 5VO, as i cant see a pre-reg.
(3)
You could also write a email to the company or call them.
If you don't speak italian: Don't worry - englisch is fine most times.
If you didn't buy the component first-hand: Don't worry - most likely they will help.
Edit As i could not find a Codice-Fiscale: The company is most likely "gonski".
(4)
According to this spec-sheet - which uses the same ISDN interface APC5568-5V - i would say with some confidence: The circuit operates from 5V DC.
According to this memory datasheet i would strongly suggest, that the circuit operates from 5V0, as i can't see a DC/DC or pre-reg on the board.
(5)
Please see @TomCarpenter's answer - just excellent.