Several things.

There is a functioning score that is part of psychological assessment. Usually a number from 0 to 100, if I am not mistaken. That is just functioning - not necessarily a degree of depression. You can be manic to the point of non-functioning.

There are questionnaires.

There is clinical observation and assessment.

Since I have not heard of the ketamine, it must not be the first line of defense.

ECT is a last resort measure, after medications have failed. ECT is not given just based on the severity/degree of depression but based on the history of medication failures. So it cannot be the first line of defense either.