The primary repository for all modern and legacy licensing information is the cloud portal. Navigate to the sign-in page.
If you purchased Civil 3D as part of an Autodesk Suite (such as the Infrastructure Design Suite), you must use the specific product key designated for that suite rather than the standalone key. How to Find Your Legitimate Serial Number
Locate files starting with adskflex (e.g., adskflex_00696100_tsf.data ).