If you are checking the status of a pending Application for Coast Guard Vessel Documentation, you may proceed to the National Vessel Documentation Center Work Packet Search Page from the US Coast Guard Maritime Information Exchange (CGMIX) via this link.
https://cgmix.uscg.mil/VDS/SearchAndReturn/Search.aspx
If you are checking the expiration status of your Certificate of Documentation, you may find the expiration date on your CG-1270 Certificate of Documentation. If you do not have your Certificate of Documentation, you may also find the expiration date via the US Coast Guard Port State Information Exchange: