And typically there is a patchfinder executable that will help you to know what you have installed.
see Updated PatchFinder for ArcGIS 10.4 available
Typically the newer versions will work for the older, but I did not test right now.
edit... I noticed that that thread only has the .tar version. for the windows version of the same 10.4
http://downloads2.esri.com/Support/downloads/other_/PatchFinder.exe
Another interesting site (although I don't see the patch finders on this...)
Esri Downloads
but lots of other downloadables there (only when logged in)