Generally the doctor/facility you're working with will request the preauthorization. In-Network providers are plugged in to the various insurer systems to determine whether or not a procedure:
- requires a preauth, and/or
- is a covered treatment for the indicated diagnosis
and will generally take care of it. Out-of-network providers are a whole different animal. If you're particularly concerned call your carrier and request the form.
There are some procedures that an insurer will not authorize unless a lesser procedure has been attempted with a poor result. For some diagnoses an MRI is a secondary step after a failed/inconclusive X-ray. Even in these cases, with adequate physicians notes and a valid concern many carriers will preauthorize an MRI without the X-ray (or whatever secondary procedure).
Ultimately, it's on the patient to button these issues up with their insurer though many providers in this day and age deal with it.