There's a few ways to find body fat percentage. You can use a caliper, use the water or air displacement methods (expensive). For not so accurate, but cheaper methods, there's just finding a list of pictures of people at difference body fat %s and comparing, biometric impedance scales, you can find you BMI (I dislike this because it just compares height and weight, which ignores fat/muscle ratios), you can take your measurements and do the math.
Best methods would be the fat caliper or the look and compare methods.
Goku - Dragon Ball Z (Otakon 2014) 0%
Colonel Armstrong - FMA: Brotherhood (Eventually) 40%
Zoro - One Piece (Eventually)
Ippo - Hajime no Ippo (Eventually)
Kaito Yashio -Robotics; Notes (Otakon 2013, Completed)
Free - Soul Eater (Otakon 2013, Completed)
Gai - Guilty Crown (Katsucon 2014, Completed)
Shu Ouma - Guilty Crown (Katsucon 2014, Dropped)