Turn them inside-out, hang in a tree, and fill with water from the garden hose. You'll find the leaks. If you have or can get one of those white or yellow tire-marking markers it'll make patching easier. You can mark the leaks and then be able to find them again after they're dried out.You may have leaks you don't know about, too. Good Luck!
"I have no idea what WW-III will be fought with, but WW-IV will be fought with sticks and stones."