Somebody once asked me about how to do that when they could not see what
they were doing.  I forget why.  My answer was, the hold the air comes out
of points at your head.  It's the only sure way I know to get it right.  In
the day, in the night, anytime at all, you can always tell where the hole
the air comes out of is.  The direction of the valve handle used to be a
good way, at least until I met somebody that separated his manifolded tanks.
The left post, of course, sticks out the opposite side.

I currently have a friend who is a very competent rescue diver somewhere up
north who has absolutely no buoyancy or trim skills at all.  He spend his
entire diving life walking on the bottom, a technique that probably works
fine where he is, but ensures you spend your dive knee deep in soft mud down
here and is the most certain way I know to ensure you don't get within
spearing distance of any fish.

Lee