Mittens-
First of all - you are a great parent!
To answer your question about the lack of outward signs let me tell you a bit about my life.
As a small kid I never played with girl toys or wanted to dress feminine; nor did I have any overt feminine desires. I did all of the customary things that boys my age did in high school (drinking, street racing, chased girls, regularly dodged the cops, engaged in some risky things to boost my man cred, and so on). When I got older I married, had children, built a drag race car, started my own business, and worked out incessantly while taking steroid precursors to build muscle. I got quite bulked up too, to the point that my legs would not fit in my jeans anymore because my thigh muscles were so built up. I carried 50lb bags of fertilizer like they weighed nothing. My friends were the studly, manly man, all male, dudebro types.
I was clearly overcompensating......
During all of my life I had no clue that I had a female brain. Once I got into my 40s the dam burst. I'm now transitioning from male to female and I know that I'm going to absolutely shock people that know me because they never saw it coming - I was the archetypical manly man guy with the perfect marriage.
So yes, your daughter could be trans* and might not show outward signs of it, or she could be hiding it from the world and from herself - denial is a powerful thing that we all experience when we find out we are trans*. She is probably working through her new feelings and might not yet feel comfortable with her new found femininity.
I'd say keep doing what you are doing, be supportive, talk to her, and most importantly - listen to her - this is a very difficult, scary thing to go through, especially as a younger person, and she really needs you right now.
~Eva