Well I have a ramped up metabolism or something so I don't have a lot of experience with gaining fat. I just have your basic knowledge on that front but I would think it would be fairly obvious. The gut is going to be the biggest clue. And elsewhere...well, muscle is hard, fat is soft
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If you meant twelve pounds in a month that's a lot. Some of it's probably going to be fat. Twelve pounds of muscle you really should be able to see. Think about a couple pounds of ground beef. Imagine distributing that over your body. Now imagine doing that 6 times....