Mirror. I'm a tie-knot snob and hate the sloppy, lopsided, loosely flapped abominations that seem to be the current trend.
A guy I had an immense amount of respect for growing up always told me, "If it doesn't look like a clip on, you didn't tie it right." I took that to heart. I use a Winsor and insist on it being perfect - an absolutely symmetrical triangle, snugged until everything lays in tidy and smooth, and with the dimple precisely centered. I can do it without a mirror but I always take the first opportunity to check and adjust so things are in proper order.
Brad