How can you tell your sails are set correctly without looking at the tell-tails or mast head fly?

If your course is set, pay attention to the motion of the boat, through your butt if you’re sitting or your feet if you’re standing:
* If the boat moves through wind waves without motion and over swells without bobbing, your sails are set well and giving you good thrust in the direction you are going.
* If the boat is heeling and bobbing, your sails are trimmed too hard or you are pointing too far downwind for the set of your sails.
* If the boat is standing up and bobbing, your sails are trimmed too loosely or you are pointing too high for the set of your sails.
* If you have significant weather helm, your jib is relatively looser than your main. (1)
* If you have lee helm, your main is relatively looser than your jib. (1)
Remember the saying “When in doubt, let it out!” and take a look at your jib. Let your jib out until it barely luffs and pull it back until it doesn’t – It’s perfectly set! Match your main to the jib, initially by matching the boom in parallel with an imaginary line between the tack and clew of the jib. Then taking into consideration twist , horizontal curve at midpoint of sail and vertical curve and outhaul, match the shape of the main to the shape of the jib, paying attention to feedback from the helm, optimizing it for a very light weather helm. The main clue will be inboard more than the jib clue more as you point higher and outboard as you run, unless you have a whisker pole. The fundamental test is the motion of the boat – Does it feel like you’re on rails, sailing powerfully through wind waves and the boat’s not rocking or bobbing? You’re doing a great job, then! I love that feeling!
Ultimately, you will feel the boat through your butt or feet, steer by looking at the horizon and get feedback from your course through your peripheral vision, picking up wave direction, wind shifts, puffs, etc., glancing occasionally at compass and telltales. You’ll get to know your boat so that you feel when the jib and main are pulling well.
(1) These assume the boat is balanced: Many are not. A balanced boat has ballast, rig, sail and rudder trimmed. There’s a lot that goes into balancing a boat, before and while sailing and that is a good bit of what I do as a rigger – Tune the boat and rig so she sails comfortably, naturally, as she was designed to.