arduino - Powering an old RC servo - Can't find the datasheet

I'm testing my arduino with servos, and my sg90 works fine for testing purpouses.

I would like to use something stronger, and my father (that in the 80s used to work with RC autos) gave me this old but promising servo.

I know I can't supply power with arduino if I want it to survive august, and I know all the servos work around 4.8-6v.

The problem is I can't really find any datasheet for this servo, and I would like to know at least the current drain to dimension my external power supply (or how can I misure it. All the wires are black, making it even harder).