Hi Jesper
First check your mainsail is not damaged. Check that the luff ropes and slides are all good and not worn. Check that any repairs are still sound. Check that all the battens are secure and not broken.
Now hoist your mainsail up the mast. Start with only a little tension in the haliard and a little tension in the foot.
Sail on a beat in a light wind with the sail sheeted home. Look up at the curve of the belly of the sail. The deepest part of the belly should be in the middle of the sail, or slightly towards the mast. If it is, your sail is OK. If it isnt, try pulling harder on the haliard. If you can get the belly into the middle, then you could use the sail for another season or two. If the deepest belly stays in the after part of the sail no matter how hard you pull on the haliard I guess you need a new one! I got a good sail at very reasonable price from Crusader sails.
Good luck! Neil
Neil Sinclair
Seal 28/27
'Andiamo of Exe'