John
Sorry to answer a question with a question, but WHICH robbe 600s? I am more familiar with the Graupner range and there are (were) about 15 of them, including the turbos.
I see that the ones I see on the interweb have a Power Speed 600/24 designation (AND a part number). In this particular case the /24 is almost certainly the turns indicating that this is a soft high-torque motor.
At the very least the motor will probably (being robbe) carry a nominal voltage 6, 7.2, 9.6 or 12 and either a Turbo or Race (or Neo) word
Then you can look up the kV in the tables and that tells you directly what the nominal speed is for any applied voltage
This hasn't answered your question, I know.
And to make it worse here is another - why do you want to know the RPM? Are you trying to match a particular prop, or achieve a particular pitch speed?
Tell us which motor, please and we will tell you the unloaded revs
andrew