Cruising boat? They are blown out
You must hoist them in a breeze to know. If they look at all old you should assume they are worthless in the purchase
calculation. Both headsails and mains should trim flat in a breeze when all the corners are pulled tight. Backstay on, mast
It is possible they are old and not blown out. In that case check the stitching for broken threads. If there's more than a few it needs to be restitched. But I've never seen a sail that needed stitching that wasn't past it's prime.
Spinnakers need to be crispy and noisy. If they are soft and quiet in the bag, like a hanky, they are worthless. You can use them, they simply have no monetary value.