Measure your height, neck and waist (and hips for women) with a tape measure.
US Navy formula, men: body fat % = 495 ÷ (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450. Women use waist + hips − neck with their own constants.
A man 175 cm tall with a 38 cm neck and 88 cm waist: waist − neck = 50, which gives about 19.2% body fat, in the 'fit' range.