Typically 'off-shore' sails will have triple stitching, seam end
patches (to prevent seam splits), heavier weight fabric, heavier weight
stitching thread, added chafe patches on the leech at reefing points,
a 3rd reef, leech purse lines that can be tensioned from the leech or
alternately from the tack, chafe patches at spreader position ..... On
some top-end off-shore sails the seams will sometimes be glued AND
triple stitched .... eg.: bombproof.

Triple stitching is the most obvious clue.



In article , Norm
wrote:

When looking at a new set of sails, how can you tell if they were
built for offshore use or for coastal use? Is there a quick and easy
way to tell?