Use the contact sheet to determine which negative to print.
Many people use a loop or a magnifier to review the print.
You can tell several things from a properly made contact proof. Cropping you might want to do. Whether the print will require more or less exposure than your normal print, based on whether it is lighter or darker than normal negatives on the proof. Whether the print will require a harder or softer paper grade, based on whether it has less or more contrast than a normal negative. To some degree, how much dodging or burning you might want to do.