Excellent topic!
By no means am I an expert on this, but I thought I would throw in my 2 cents on the topic.... and invite any corrections on my assumptions. And all my observations are in the domain of, "What do you do if you don't know your True Will yet?"
I've gathered that doing your True Will has a certain amount of "joy" in it, although it can also be hard, especially if one is overly identified with their personality. I mean, it is what we were "meant to do" and "who to be" right? And in addition to joy there is a "rightness" and added energy present. Any hardship present while doing your True Will would be caused by the personality "putting on the brakes" so to speak, or going against the natural flow which is the True Will (not to be confused with what the personality calls 'natural' for what that is really just familiarity.)
So, it brings up the question of how do you tell if something in the moment is your True Will or not? The practical advice I once heard a certain wise man give ( ) in regards to this is that at any given moment, choose the thing that rings true in your gut as being the most right,... that will be the closest thing to your True Will in that moment.
Also, if something is hard or scary I ask myself, what is the quality of the hardship and fear? Is there a thrill there also or an excitement? Then it might be that my personality has to let go of something and embrace more of my true self. Is the hardship just draining and drudgery? Then maybe its not my true will but rather some lesson on the path.
The above might be rather simplistic, however I think that most human beings get very caught up in automatic reponses and ways of being. Also, as human beings we are conditioned to identify with our personality "T-shirts." So it takes a multitude of small steps, I believe, of bringing more consciousness into our thoughts and daily choices. This is the path or 'moment to moment' practice of discovering one's True Will, in my opinion.
I also believe that if we leave out this practice, we can do any amount of Star Ruby's, LBRP's, Reguli, what have you...it won't get you squat in the area of discovering one's True Will. Of course, I'm also of the same opinion as Regardie in that all magician's should get therapy!
Ok, back to work for me...thank you for bringing up this topic, it seems to have brightened up my previously miserable day!