The plastic sail slides deteriorate from UV exposure. It's not a big job to replace them.
As for the main sail I couldn't say. The best amateur test I can think of is to raise the sail, put strong halyard tension, outhaul tension, Cunningham and some vang and sail up wind in 10 to 12 knots. If the maximum draft of the lower 2/3's of the sail is more than 50% aft, the sail is shot.
You can sail with shot sails if you don't care about performance. But I think sailing is a kinetic sport and performance is more fun. Not only fun but the reason to go out.