If he used 3/4" OSB (oriented strand board) with the tounge and grooves, AND left AT LEAST 1/8" gaps between them for expansion, AND put the "waxed" side up then you are probably fine. Also, some effort should be made to eliminate standing water, such as squeegee the water off or even drill small holes where it might collect. I have found that good quality OSB underlayment holds up better than regular plywood underlayment when exposed to the elements. This is because of the waxy coating that keeps it from getting too wet. If he didn't use the right stuff then you will be able to tell that it is swelling and maybe even puckering up at the joints if he didn't allow for expansion by leaving joint gaps.