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.
Free - Soul Eater (Otakon 2013, Completed)
Goku - Dragon Ball Z (Otakon 2014, Completed)
Teen Goku - Dragon Ball Z (Otakon 2015, completed)
Powerline!Max - A Goofy Movie (Otakon 2015, completed)
Cafe!(neko)Kaneki Ken - Tokyo Ghoul (Otakon 2015, completed)
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Author of the "Getting Fit for Cosplay" Katsucon panel.