You can deduce the position of phases in the sockets of the power panels by comparing the voltage on them using a normal multimeter (you have to see the nominal voltage Vn between phase and common and the phase-phase voltage Vp = √3 Vn between two different phases).
Usually you have something like this:
On the left side you can see a normal (in this case italian) single-phase plug and on the right three single-phase Schuko sockets.
Each one of them is connected on a different phase of the system to have a more balanced power's absorption.
In some applications you'll have a three-phase (five poles) socket too, in this case you can deduce the position of phases, grounding (G) and common (C) conductors the same way, using the multimeter.
For example you can see a usual disposition in the picture on the right.
Now all you have to do is to plug the impmeter between a phase (P1, P2 or P3) and the common to measure the single-phase ZSC or between two different phases (P1-P2, P2-P3, etc) to have the two-phase ZSC.
That's all for now. Bye!