A big study looked at bipolar disorder in the Amish people of Pennsylvania. The results even were pointing to a particular chromosome. But After the results were in, several of the subjects who had not shown symptoms developed them. It threw off all the results. Studying depression in this way is even more difficult.
Manic symptoms are often pretty unmistakable.
Many people have periods of low mood who don't seem to have a mood disorder. All with different kinds of combinations of depressed and elevated mood episodes. It involves stimulating the brain tissue with a magnetic field. It appears to have the same effect as ECT but is much safer and the patient does't need anaesthesia. It is still an experimental techniques but is very promising.