How soon can a psychotherapist determine depression?

Interesting question: Sometimes it can be easy and accomplished in a diagnostic interview and mental status examination. Sometimes people "mask" their depression and it may not be clear until a therapeutic relationship and trust has been established or on testing if there are other psychiatric conditions which might have depressive features. Also not so simple: medical eval is needed to rule out medical causes.

Quickly: It is very easy to diagnose. Generally a competent psychologist or psychiatrist can diagnose it in a single consultation. But it is important to understand what is causing depression, and there may be more going on than depression alone, so a thorough evaluation is always in order.