Just call up their office and ask. I asked three of my doctors (my pcp, my neuro, and a surgeon) and my pcp and neuro both said no because I was a patient of their practice and having me as a shadow would affect their objectivity. My surgeon, on the other hand, throws me a few hours of OR time here and there (he would throw me more, but of the three surgical centers he works out of, only one is easy to sneak me back) and has been introducing me to other surgeons. My neuro introduced me to another neuro who I will be shadowing for a few days. My PCP hasn't helped much.
I have learned one thing, telling people you are a premed who overcame a medical phobia (or to leave out the premed part and just say you are overcoming a medical phobia) will get doctors more interested in letting you shadow. That is my experience anyway.