I would mainly want to know if they accepted me. And for me personally, my being trans and my transition, etc is the hardest thing in the world for me to talk about. So I'd just want to get on with another subject as soon as possible. Everybody might not feel as awkward about it as me though. The less discussion, the better for me.
So I'd suggest just seeing how they're responding.
If they did suspect, it might make me feel better that they really 'got' me all this time. But saying you suspected or that you didn't might make some people paranoid about how well or not they were covering themselves. We have a lot of pressure surrounding 'gendered' behavior. Before we come out, there was a lot of pressure to behave like our birth gender, and some of us worked very hard at it. After we come out, there's a lot of pressure to examine our past to prove we were manly/girly enough.
So some might like to hear whether you knew and others might not.
It depends on the person and how comfortable they're seeming.
Good luck. It's awesome you're thinking about how best to support your friend.
Hopefully, you'll get some better ideas from other posters.
