in daylight, stand up inside it with head at the feet end, i know this sounds daft, but you can inspect it much more thoroughly than either filling it with water, which is a massive task, putting it in the bath, or trying to run a torch all round it on the inside. this way any small wholes will be obvious, you will also have a giggle at how mad you must look. i found loads of pin size holes in mine using this method that i couldn't see from an outward inspection.