DO NOT ATTEMPT TREATMENT WITHOUT LICENCED MEDICAL CONSULTATION AND SUPERVISION
This is a public discussion forum. The owners, staff, and users of this website are not engaged in rendering professional services to the individual reader. Do not use the content of this website as an alternative to personal examination and advice from licenced healthcare providers. Do not begin, delay, or discontinue treatments and/or exercises without licenced medical supervision.
An orthodontist will eyeball it. They are very accurate at calling Intermolar width (IMW). Some will actually measure it. To measure yourself, I prefer the celery method... http://orthotruth.com/whats-up.html
As you undergo correction in the near future, please consider keeping records for your own sake and for others. Pictures of dental impressions, scans, medical reports reports can be very helpful even with all personally identifying information blocked out.
Your input could help many, many people