It's a good point to ask for your PD as well when you get your eyes tested. Often they won't put this on the script but you should ask whilst you are in the appointment.
And if they won't tell you just go to the warehouse and buy a ruler for 50c.
Ha, clearly contacts have a ruler pdf you can download and print! It is surprisingly difficult to measure precisely and as has been mentioned, it changes slightly depending on how close you are focusing at the time.