A "regular" eye doctor (optometrist or ophthalmologist) can make the measurements that can tell you if meet the requirements of "legally blind." However, they do not "declare" you as such.
Legally blind means that with best correction, your visual acuity is worse than 20/200. You would startwith an eye examination to determine if that was the case. Next step, if you are applying for disabilty or rehabilitation services, you would make application. They will send paperwork for your eye doctor to fill out to attest that your vision impairment is worse than 20/200 with best correction.
They may ask you to have another exam before making their final determination.,
licensed optician who helps fill out all the paperwork...