Not what you asked exactly, but response to drugs sometimes leads to an (objective) differential diagnosis: if someone is depressed and flips out in a manic attack when starting antidepressants... that's not Major Depressive, that's Bipolar.

How do you objectively verify "manic"? You can track hours slept. No one fakes sleeping two hours a day for a month while having, say, good motor skills in the daytime.

This matters because recurring brief hospitalization is necessary for some people to get back on their axis. This is NOT "sanity days" or "mental healing", it's a clear cut medical issue.