thats the filter side, the housing side is a bit different and has metal tabs sticking up which are for the seal to hook onto (on a gen vag cooler anyway) you'll know which way is right when you try to fit the seal, it should sit inside a recessed bit not on a flat bit
edit: filter housing side, notice the metal tabs and larger recessed area for seal to sit in
edit2: if you have an oil filter lying about, test fit it to the cooler, you'll soon realise which way is right when you see where the oil filter seal sits
