To find out the peak current drawn by a motor you can measure it thus:
disconnect motor from the circuit.
Use multimeter on resistance range across the terminals. Take a few readings, rotating the shaft a little between
each reading. Discard all but the lowest reading.
Measure the resistance of the multimeter leads by shorting them together, subtract this from the motor reading
to give a best estimate of the winding resistance.
Say this gives 1.6 ohms (for instance). At 12V the motor can pull 12 / 1.6 = 7.5A. [ insert your own figures here of course! ]
If your supply cannot supply this current its voltage will dip momentarily as the motors start moving, or if they stall.
[ Adding a large electrolytic cap can reduce the dipping associated with starting ]
If the supply dips then you can't share it with the servos and other parts of the system - it may still be
perfectly adequate for powering the motors under normal load.