Dry the valve; then wait a few minutes and check it again. If it is wet with even a drop of water, you have a leak. Turn off the water and repeat the process. If the line continues to leak, the water is coming from the tank or the hose connections.
Often the problem is simply a matter of turning the valve to the correct position. Turn the valve to the off position, turning clockwise until you meet resistance. Open the valve again by turning it 90 degrees to the left, so that it is parallel to the water line. You may feel the valve click into position. Check the valve later to see if this corrects the problem.
Purchase a replacement valve at the hardware store. Turn off the water supply to the house and flush the toilet to empty the tank. Place a dishpan under the valve to catch the water in the line. Unscrew the valve from the water line on either side. Wrap the threads of the new valve with leak-stop tape and screw in the new valve. Turn on the water and check for leaks.