Measure you PD
If your PD isn't indicated on your prescription, you can easily measure it yourself. Watch this instructional video:
Measure the distance 3-4 times to make sure it's accurate.
Normal PD for adults is: 54 mm to 74 mm
Normal PD for children is: 44 mm to 53 mm
Your optometrist also probably has this value in your record. Another option, therefore, is to give them a call and ask for your PD, and you'll get an accurate measure.