If someone questions you, just say "I'm transitioning." Simple, brief, to the point. Hold your head up high, behave like the person you want to be, with confidence.
At least that's what worked for me. Voting in person in my precinct years ago, when getting a puzzled look from the voter check in staff, that seemed to satisfy their curiosity. Adding "I haven't changed my legal name yet" helped.
Since I've been working as a poll official for at least 3 major elections, we really don't have much time to question a person's gender. And unless someone has had full facial feminization surgery, the basic structure of a face doesn't change all that much through transition. One thing I'd recommend is holding make up to a minimum for the MtF transitioners. There is nothing in my poll manager training manual that mentions gender matching, and it's never been brought up in class room training.
As an example, during the spring primary this year, I handled an attractive young woman. It wasn't until handing her DL back that I noticed the "M'.
Now that I have changed my name, and IDs, life is much easier.