If you look into the bilge and see water after you take the boat out pull the fuse if you have an auto bilge, if the bilge works by switch don't turn it on. Take the boat home and dry the hull. Then look below and see if you have leaking rivets or a crack. If no leak then tilt the front of the trailer up and inspect for leakage. I don't recommend putting more than a few inches of water into the bilge with a garden hose as well. A good amount of water will stress the aluminum at all the trailer contact points.
A second thought, measure the amount of water in the bilge, if there is less when you get home you know it isn't coming in from rain, waves, swimmers etc. I am assuming you don't have a long drive back home. You don't really want a copious amount of water in the boat when on the road for a long period. If you have a long ride back park the boat at the Marina and dry it off.