There was an excellent fact sheet on telling up/down sides on cotton/cotton blend batting in the ACQS (American Professional Quilting Systems) newsletter dated Nov. 2010 which I found somewhere on the internet some time ago.
To make things interesting, they indicate some battings "don't have a right or wrong sides" which explains why some folks never have a problem. They do give this hint : often, a pin inserted into the cotton batting will go into the fabric much easier from the "up" side than the down. I have found this to be a good indication....