If the drain is in the middle (as is the case with the tub in the master bath), you can (generally) look at the shape. In general, tubs have a flatish side and a curvy side, just like a capital D. If the flatish side is toward your right, it's a right-handed tub (and vice-versa)! Indeed, this is how our tub-o-choice is arranged (although we have a curvy side and a less curvy side):
So then the question is: Do we want a left-handed tub or a right-handed tub?
In general, your back relaxes into the curvy side of the tub (the head) and your feet dooble down toward the flatish side of the tub (the foot). In the master bath, there's a window toward the eastern side of the wall for the tub area, so it makes sense (to me at least) to face that side and be able to look out the window (to see if the cable guy is peeking in on you...) rather than looking into the wall (although it will be a fine wall).
Fortunately, I remembered that I had actually sat in one of these tubs eons ago, so I dug up the photo to see how I sat in the thing, and this is what I found:
Comfy! (Notice the look of abject content.) And I was lying in it with my back on the "flatish" side. After a quick search of the inter webs, I verified that this is the proper seating situation. Soooo..... the head of the tub is on the "flatish" side (glad I sat in it!). So what we really need is a right-handed tub:
Which, to my eyes, actually works better geometrically. In terms of where the faucet controller goes, I'm thinking right above the overflow drain.
I was concerned about the ceiling truss running right up the middle of the tub area and interfering with the placement of the ceiling spout, but the manufacturer suggests placing it somewhere in the back quadrant away from the head of the tub: No need to center (and in fact shouldn't be centered).
Chosing the hand of the upstairs tub was easy because it tub placement already done by the architects and we're using a more standard tub:
Makes sense to put the foot of the tub on that side since that is where all the plumbing is.
Two righties tighties it is!