With experience, you'll know by eye if the negatives are good or not. If you want precision confirmation of your negative's quality, get a densitometer and then you can quantifiably measure the range of density from your deepest shadows to the brightest highlights. Being able to quantify the density range on your negatives by itself doesn't have meaning - different printing processes require different density ranges so what may look good for a silver gelatin print will be weak for a salt print, or too contrasty for a cyanotype.