Accuracy is a product of accurate reference information in the first place. Many things out there only have screencaps and pictures and suchlike, which makes it a 'best guess' when it comes to making a model of it. If it looks like it, then you're doing well. Get as many photos from as many angles as you can and keeep referring to them as you work.
For well known stuff like the various Enterprises you can download the blueprints or orthographic drawings of the ship in question, and most 3D programs I've worked with allow you to place a reference image in the background of your modelling window so you can basically trace the outlines. In Lightwave you have the front, side and top views of your model as you work, so when someone like Reverend makes a 5-view layout of a ship I then chop it up into it's seperate views and load the appropriate pictures into the view windows. I then 'trace' the blueprint and out pops an accurate 3D model.