Even though you may have just purchased a gun, there is no way to tell how long the tritium vials that are in the sights have been sitting around before they were installed in the sights.
Strictly speaking you are right, however, at one time it was easy to identify what is at least a warranty countdown starting point on a Trijicon- the date was often screened on to the side of the rear sight (and sometimes the top of the front sight).
Nowadays, with Trijicons, the sight must be removed to identify the date of manufacture, but the information is usually there to be found.
In the case of Trijicon-equipped Novaks and Heinies, Trijicon has the date of manufacture screened on to the underside of the rear sight (used to be right there on the side of the sight itself).
When HESCO owned Meprolight they used a two-letter code stamped into the underside of the sight for the same purpose.
I do not know if this is still the case with Meps under Kimber ownership.