Take your board out on a quiet street, do a few ollies, and listen for the "snap" that a nonwaterlogged board makes. The sound should be crisp and clear. A waterlogged board will feel almost like dragging cardboard as you elevate, and the sound will be dull. It may also make a "creaking floorboard" sound.
Look for a warp or dip in the deck. Use a level to check whether the deck is straight. Examine the grip tape to see if it is fraying, peeling or discolored.
Pay attention to the state of the wood. Signs of waterlogging include chipping and splitting. If the glue loses its tackiness, all seven layers can crack and the board may split in half.