You want to be on the latest version of SharePoint.
There are multiple ways to get it:
❌ Check SharePoint DLL version
❌ Check http://<CentralAdminSite>/\_admin/FarmServers.aspx
✅ Check http://<AnySiteCollection>/\_vti\_pvt/service.cnf
See Get SharePoint version number of your platform quickly for details.
vti\_encoding:SR|utf8-nlvti\_extenderversion:SR|16.0.0.4327
Figure: On SharePoint 2016, this is what you get for intranet.ssw.com.au for example