Um, I think I understand the question:
If you're shadowing/volunteering an optometrist for experience, then nothing is needed for "proof". The experience will help you during interviews, and for you to get an overall feel for the profession. You just note that you shadowed/volunteered for how ever long, and where, in the application.
If you want a letter of recommendation, then that's a different story. In this case, the "proof", is a letter that they write about you, and send off to the school. To get a good letter of recommendation, you should get to know the optometrist well before asking for one. Generally, to make a good impression, you should show interest in what he/she is doing, and try to be inquisitive.
In general, whenever you are with an optometrist, ask anything pertaining to the profession, from their college experience, job experience, machines they're using, diseases they're diagnosing, etc.
Hope that helps.