If there is no name plate, you won't be able to know the rated voltage too. In that case, you may have to guess the approx. HP depending upon the size of the motor, select appropriate auto transformer and then run at no load by slowly increasing the voltage till it reaches steady state. I am however still not sure if this will give you the correct rated voltage. There are standard ratings of three-phase 380V, 415V & 440V. I do not know if differentiation between these is possible, because the difference is very small.
You will operate the motor with auto transformer at no load increase the volt step by step and at each step record the motor speed until the speed go to be fixed at this moment this is your rated voltage.
For motor current, you will operate the motor at it's rated voltage which we get above then loaded the motor gradually and record motor current and be careful for loading motor current will be proportional to the load until moment will increase with slightly higher, so be careful you at the rated motor current.