Here's where two tools come into play. The first is the bathroom scale. If you're lean, you should see the numbers increase. If they don't, you aren't eating enough protein and calories to push up your muscle weight. The second tool is a skin caliper to measure your bodyfat percentage. This will tell you how much of your weight is fat and how much is muscle mass. What you're looking for here is a decreasing bodyfat percentage, even if only slightly; any change in the negative direction is promising. Don't try to use the caliper by yourself or have just anyone take your measurements; you'll need to find a professional, either at your gym or in another health and wellness-type setting, who has experience administering such bodyfat tests.