Another MD here- this sounds simple, but most people don't realize it's legal to practice without residency training. It is (at least in certain states), and SUPER scary. Never, ever go to any doctor that isn't board certified in their specialty. (Residents excluded, because they are practicing under direct supervision from board-certified senior doctors.)
Beyond that, if you don't understand what your doctor is thinking, ask. If it doesn't seem reasonable, get a second opinion. A doctor who knows what they're doing should be able to explain in layman's terms why they want to proceed in a certain way, and it's okay if "I don't know exactly what it is" is part of that. For example, it's totally reasonable to say, "I don't know for sure what your rash is by looking at it, but most rashes like this turn out to be an allergic reaction to something, so let's try a steroid cream for a while. If there's no improvement in a couple of weeks or it gets worse, we'll refer you to dermatology for a biopsy."
Unfortunately you also can't necessarily go based on outcome either. Working in an ER I hear people complain about the way their doctors handled their initial symptoms ALL THE TIME. Frequently the primary doctor started a very reasonable workup and things just got worse suddenly or it turned out to be a zebra (very rare condition that it doesn't make sense to suspect right away.) In those cases I have to explain that I would have done the same thing, but now that xyz happened we need to do some additional testing. Other times I'll look at somebody's med list and see they are on a totally crazypants set of medications for their chronic problems, but they are totally happy with their primary doctor and have no idea.
Some of this is personality fit as well. I tell people to lose weight, stop smoking, etc. If you weigh 500 lbs and come in complaining your knees hurt, I have to remember the appropriate medical phrase for "no shit." I absolutely believe the serious side effects and risk of life-ruining addiction to narcotics trumps your unwillingness to tolerate minor pain. I won't prescribe you oxycodone for your sprained ankle. Certain patients would be very unhappy with me for that reason and should find a different doctor.