It does take practice, but soon enough you'll KNOW (without looking) that you're on the right or wrong diagonal. :) You can just feel it, it won't feel right. It'll be off-rhythm.
When the outside shoulder goes forward, you rise. It's OK if you don't get it right all of the time (nobody's perfect!) usually I don't ever glance, I can just feel it, yet once in awhile I DO glance for a sec. ;) That's okay.
Yes, to change your diagonal (which is also good to know) I just sit an extra time. ;)
Ride more, worry less.