Calling and asking if they have exclusions for transgender care is a good start. What I did was use the online messaging option and list the specific procedures I wanted to have done, and asked what kind of coverage I could expect for those things.
They got back to me about a week later with a complete break down of what I could expect to pay in and out of network, what % of the expenses they covered and what I would be responsible for, who I could go to that was in network, etc. The only thing you may find with insurance is that you have more limited options of who you can go to. So I wouldn't get your heart set on a particular surgeon until you know who your options are.
To get them to cover it, I needed a letter from my doctor saying I was healthy enough for surgery and met the WPATH guidelines, as well as 2 letters from mental health practitioners saying that SRS was medically necessary and that I met the WPATH guidelines.
