If you have the original USB or DVD, the serial number may be on a label on the box. For downloaded files, check for a MID.txt file in the setup folder; the first five characters of the part number often indicate the product key. Important Legacy Considerations Autodesk Product Keys - JTB World

If you are installing Civil 3D 2016, you will likely need one of the following Product Keys depending on how the software was purchased (standalone or as part of a bundle).

If the software is currently running on a machine but you need the code to reinstall elsewhere: