Welcome to the Forum!
You will need some sort of supervisory software which could connect to and audit the application file running on the CPU controller.
For Allen-Bradley manufactured CPUs, FactoryTalk AssetCentre with Disaster Recovery Agent (http://www.rockwellautomation.com/rockwellsoftware/assetmgmt/assetcentre/overview.page)
is the commonly used tool.
However, the "time stamp" of the detected "Change" will be determined by the FTAC "auditor" utility schedule; there are no CPU "on-board" (or RSLogix 500 software) provisions for logging the time of application downloads.