Your question made me think, so this is what I've come up with. Thank you!
1. My own doctors. They know me as an individual, and they have the expertise.
2. My common sense, life experience, and ability to apply critical thinking and understand myself.

These two things combined usually work well for me, but I might add:

3. Friends and family who have been through something siimilar, particularly if they are not bossy but willing to be a sounding board!
4. Trusted medical information sites like Mayo. My husband was trained as a librarian, and very good at evaluating what is legitimate info.

What I don't use: any thing from media, snake oil sales people, too-good-to-be-true "cures," or anything that is being promoted and sold hard. That includes drug advertisements or goofy advice from friends.

Also, it helps me to not expect to get the perfect information at all times. Medicine is indeed an art as well as a science, and life is full of gray zones. I try to make good decisions while accepting that I'll never have a complete picture.

I like your question because it helps us all think about our own process.