There is a test called a BDI test which helps to determine if you are depressed or not.
Im not sure if your GP can give you this test but I was referred to a psychiatric nurse by my GP and I got it there.
Its simply just answering questions about how you sleep, your day to day mood, apetite, motivation and energy levels , things like that.
As louisa said, if you've been depressed before then you'll probably be able to recognise the symptoms.
You can even go to your GP just for a chat about how your feeling and take it from there.
All the best xx