As I learned from my 'spy' thread, this will likely be something specific to you both. There, unfortunately, isn't one full-proof method to figuring it out without revealing yourself.
The below suggestions are good, but I'd wager don't stray too far from your normal activities. If she's the kind to think something is amiss, she'll dig deeper.
Also, understand that people can change. So, even if she appears against transsexuals at first, that may not be the case forever.