Need help in calculating voltage

Hi there ,
I am designing a sum bot robot , but one question I was asked by my professor is what is the voltage a cross the DC motor when it is operating at full speed
I am using 4 solarbotic GM6 motors and I am connecting the motors to the ATmega8 through an H-bridge .......
Please if you have an idea let me know ..... I need my design to be Approved......

That will be on the motor's data sheet, or printed right on the motor itself.