--on what they're trying to diagnose.

Process, I think, they can determine pretty accurately with the proper equipment; i.e., whether one is undergoing an anxiety reaction, whether one has achieved a relaxation response, etc. The REASONS for these, though, would be much harder to discern, unless one was having a directly conditioned anxiety response to a particular stimulus--a phobia. (In fact, the most common use of biofeedback has been by behavioral psychotherapists in controlling phobic anxiety responses.)

One can develop a surprising degree of control over a lot of autonomic responsivity that many think is beyond conscious influence--blood pressure, body temperature, sweat response, adrenaline release--but it takes time and a good degree of concentration; the more distractions/multitasks are in the surrounding the environment, the more difficult this is to do.