Presumably, by comparing how much time a teacher takes to bring them to a similar level of recognized mastery-usefulness in both rationalism and some other skill / field of knowledge where the teacher is reliably competent and equally knowledgeable in both fields.
I'm just throwing up a conjecture here with the goal of spurring on further thought, though, as the question intrigues me.
Recently, LWers Will Ryan and Divia Melwani (now Will and Divia Eden) were married, with Eliezer Yudkowsky officiating.
I've been to 40+ weddings in my lifetime, and this was my favorite ceremony yet. Here is the video, and below is the transcript of Eliezer's... what's it called? "Blessing"?