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.
Undercover Steve Rogers - Captain America. (Awesome Con 2017, Summer Southern MD Cosplay Meet-Up)
Organization 13 Roxas (DAS Dover Halloween Party and Mini-Con 2017, Cosplay Judge)
Xe Yui/Lord Grim - The King's Avatar (Katsucon 2018)
All Might - My Hero Academia (Katsucon 2018)
Goku (Ultra Instinct) - Dragon Ball Super (Katsucon 2018)
Possibly? Male!Megumin - KonoSuba (Katsucon 2018)
Southern Maryland Cosplayers Group
(Anyone can join)