Measurements would be the go I guess but I've never been into that much.
The other is bodyweight accounting for bf%. i.e if you WERE 200lbs at 15% and now your 210lbs at 10% then you know that you previously had 170lbs of muscle and NOW you have 189 lbs of muscle.
Personally though, I base it on my lifts. If I'm getting stronger for reps then as far as I'm concerned I'm making gains.