Welcome to the forums! Depending on the parts in question, scratch-builders are usually working from either scale drawings found in numerous publications, working from scale-accurate parts/representations such as resin or styrene molded parts, or from actual photos of the real thing. In some cases lots of details are available from surviving museum pieces where you can get exact measurements, in others you have to do some guessing or detective work to get it figured out. There isn't any one-size-fits-all answer to your question other than research, research, research! 