There is reference points on the robot for calibration, including one on the base.
Page 123 of this manual lists them:
ABB does not calibrate the robot by finding the x,y,z position of the effector hand by direct measurement. They calculate it by zeroing the joints rotation sensors, and using that to 'blind reckon' x,y,z position from the 'home position'
The problem is that the manufacturers calibration system requires the manufacturers special pendulum calibration tool. More details on the mark locations and the calibration process is this manual:
You may be able to rent the calibrator from ABB.
If you don't have the calibration tool, then you may be able to reference using a machinists level, a accurate ruler, and a fair amount of patience!