It is not something I would personally do. If they haven’t told you yet, there may be a reason, dysphoria or otherwise, and I’m sure you wouldn’t want to make them uncomfortable. I would just ignore the want to know for a while and wait for them to tell you by their own choice, or it to come up in a conversation. If they are already speaking of themself before their transition, that may be a sign they are comfortable telling you, but you shouldn’t worry if they don’t.