With some people I tried to cover the Trans* subject in general conversation first. I.e. Mentioned the Wakowski sisters (were brothers) when talking about films and The Matrix to gauge if they had any strongly negative feelings.
Otherwise, there is not really any easy way that I've found aside to just tell them, and accept that any initial reaction is a valid one, and then go from there.
I've tried to do it in a neutral public space where I've been able choose.
If, how and when you do it, I hope it goes well for you. X