First of all, a receipt of the customers purchase is always the best way to check if a device is still under the warranty circumstances.
However, if the customer lost his receipt. There are two more ways to check the age of the phone:
Date of manufacture:
Every device has its manufacturing month and year lasered onto it. On the underside of a base station you will find a code starting with CT/ and followed by two or four letters/numbers. At handsets you will find this lasering inside the battery department.
The four numbers represent the calenderweek and the year when the device was manufactured. So CT/2015 says that it was produced in the 20th week in 2015.
There is also another manufacturing code. If you find a code like CT/KN. It would mean that the device was produced in November 2018.
The third way to check the age of a handset is to ask for the runtime of the handset. This can only be used for checking if keyboard and display still work.
In idle mode press the "menu" button followed by * # 0 6 #
Ask the customer for the first five numbers of the second line. It shows the time in hours from first starting the handset. 17520 for example would be two years.