How Were You Diagnosed?

I know I need to talk to someone about this. But is it normally diagnosed by blood levels in winter of melatonin or serotonin? I used to want to go back to sleep quickly after starting my day job as a delivery driver. I felt so fatigued I had to come home for a while. It's been easier as I've not had my own car in a long time. But I almost have an association of being tired with driving so I have anxiety when I try to drive other's cars in a cloudy time of day with little light.
I've been in OR most of 24 years. I'm needing to move to a much more winter sunny climate, and a dry one would help me with health as well. I hope to get to maybe Las Vegas soon. I'm not sure if SAD exists that much there?