Measuring girth

I've discovered that the easiest way for a guy to tell if he's gaining weight is to buy jeans that fit him perfectly, and then monitor over time if the jeans are getting too snug or loose. Just this past week or so, unfortunately, I've noticed that my new jeans, bought two months ago, are definitely starting to get a little snug. I was so happy to get down to a 31 waist size, but now I could stand to have a 32 at least. Looks like I need to cut out the extra carbs and sugar and the late-night beer once in awhile!