To perfectly fit a men's suit, the neck, chest, stomach, waist, hips, shoulder, bicep, wrist, crotch, knee, vest length, jacket length, sleeve length, pants length, inseam, thigh width, and half hem must be measured. It is also possible to measure an existing suit, says the suit measurement guide at ModernTailor.com
TheArtofManliness.com using it's recommends using its visual guide to learn to identify a perfectly fitting suit.
Not all suits are created equally, however, and the perfect fit of the suit depends on the desired cut. A slim fit suit will feel tighter to the body than the roomier classic fit, for example. The choice of vents will also change the fit of a suit.