WaterNomads said..
Close the vent plug, place the board in the sun for a few minutes. With some soapy water you can check if there are any air bubbles coming out around the vent plug. If there are, air comes out which means water can come in. No bubbles means you are probably ok.
Yeah best to open vent and leave board in cool place eg inside overnight. close vent then move to the sun. But I would give it 15-20 min to heat up then go with soapy water