Either you make an evaluation on your speed, pointing, control in gusts, subjective looks and feelings, or you can download accumeasure and try do do an analysis. This is easiest to do if you have pictures from when the sail was new, taken from the foot/boom and up.
http://www.ukhalsey.com/accumeasuredl.aspIf you want some opinions, post some screenshots from accumeasure here, send them to your sailmaker (or the original sailmaker) and get several opinions.
Lay the sail on the ground, remove battens and see if it have stretched, like Hobie1616 suggests. If it have gone that far, you will probably want a new one. It is very subjective though.