How would a psychiatrist vs general physician test for depression?

Depression Screening: Psychiatrist are specialists with added training experience and board certification to diagnose, treat mental health issues.Depression screening can be done with assessment scales/screening questions, but there are a lot of symptoms that don't mean depression or can be part of depression.Psychiatrist knows how to find those details and come up with the right diagnosis and treatment.

GP vs psychiatrist: Most depression is treated well by general practitioners currently. The difference is that your gp is responsible for all medical problems, and is limited by this. If you don't respond well to the gp's treatment or if your symptoms are complicated, or you have a family history of mood swings, consult a psychiatrist.

Psychiatrist vs GP: Drs. Munir & knox have given good replies so far. I'd like to add that most gp's do not have the training or time to do psychotherapy for depression, and this is an essential part of treatment. Psychiatrists are trained to do this, plus how to differentiate psychiatric conditions which complicate or are not just simple depression, and also how to augment antidepressants that aren't working.