For all we know Putin died last year during Prigozhins revolution, and the whole country is being run by a group of Oligarchs who just wheel out their supply of Putin doubles when required.
You’d monitor and observe the reactions/interactions with the daughters and sexual partners. That would tell you if he was assassinated and run by a body double.
The secret services of every country know the relationship between the family and partners of leaders is a good thermometer of psychological state.