I used to have the same doubts as you do (I also really needed to know!!), so I decided to try something. It may work for you too.
I made an effort to make new female friends outside my social circle. These women had never met or seen me before. I spent time hanging out with them in both group outings and one-on-one meetings. I spent entire days with them, just the two of us, going out to museums or whatever. Just enjoying a day out together as women.
After we'd gotten to know each other across multiple outings over the span of a month or two, I found a time when just the two of us were alone in a car. And I told her I'm transgender. I figured the best way to judge a genuine reaction would be in private, in person, and watching the reaction on their faces. Anyway, when I told her, she was truly surprised and had no idea. I asked her, "but didn't my voice give me away? or the way I acted? or the way I looked or dressed?" And she said no. She had absolutely no idea I was transgender, even after all the time we had spent together.
I did this a few more times with other female friends around my age, and each time I learned I had been passing flawlessly. After doing this several times, my mind was finally put at ease and I stopped outing myself to my friends, haha
