Here's another website I found called AskMen.com. They interview an apparent denim guru called Jarrid Adler, who has worked with denim and the clothing business for over 15 years. Here's his answer to the question: How would a regular consumer be able to tell a fine-quality denim from a lower-quality denim?
"For the layman who doesn’t know anything about fabrics and quality, they can usually tell right away by the feel of the fabric. That’s a really good indication. If you feel denim that’s soft and has a certain moisture to the fabric, which usually means the fabric was made with good-quality cotton, even if the denim is raw and unwashed. The touch is a big factor in determining the quality and look closely to the fabric of the denim. There are so many processes: traditional is what is called a right-hand twill, there is left-hand twill, a ringspun, a double ringspun, a cross-hatch, a broken left-hand twill, a double ringspun... Basically, look for interesting characteristics of the fabric and the more interesting the fabric the better the quality."
"Then there are certain things like stitching on the side-seams -- there are basically two ways to sew the side-seam: over-lock stitch or open seams. The open seam is the process where the seams on the outside of the legs are sewn and pressed flat for a more comfortable feel. In order to do this procedure there are three steps -- sew the ends of each side, sew the two pieces together and press the seam flat. The over-locked machine sews the two pieces of fabric together and cuts the excess all in one step, making the process easier and less expensive; however, it leaves a bump on the side of your leg."
"Other quality notables are copper buttons and rivets, doubled-belt loops, two-piece waistbands, and back pockets that are contoured to the bum."
"The truth of the matter is, at the end of the day, you want something that fits you well and looks nice on you. There is always price barrier, but from a certain price point, if you can find a jean that really fits you, makes your bum look nice and you’re comfortable in, that’s usually the best pair of jeans for you."
You can read the full interview here. It's full of really good information about raw denim and such.