Measuring Current rating
Without a schematic or a parts list, there is no definitive way of determining how much current a supply salvaged from old equipment can deliver.  You can try to determine the part numbers of such components as the power transformer, rectifier diodes, voltage regulators, etc. and look up their specifications but this will only give an indication of how much current the supply was designed for.  For your application, I would suggest first measuring the output voltage of the 12 volt output (assuming there is one) and then connecting one of your motors to it.  Measure the output voltage again.  Note how much the voltage drops and observe the motor for a period of about one hour.  Note if any components of the supply seem to be getting hot (be careful not to get burned).  If all looks good (the voltage doesn't drop by more than 0.5 volt or so and nothing gets too hot), try connecting another motor.  Repeat this process until all 4 motors are running.  One hour should be long enough for all of the components to reach their equilibrium temperatures.  How much the output voltage can drop and still run your motors is a function of the motors but I would assume that if the voltage drops below 11 volts, the supply is probably not adequate.  If the supply passes this test, then it is probably OK to use.  But, again, there are no guarantees when using salvaged equipment with no documentation.  Good Luck.