From the BBC:

In the majority of cases in hospitals, people are pronounced dead only after doctors have examined their heart, lungs and responsiveness, determining there are no longer any heart and breath sounds and no obvious reaction to the outside world....

Many institutions in the US and Australia have adopted two minutes as the minimum observation period, while the UK and Canada recommend five minutes. Germany currently has no guidelines and Italy proposes that physicians wait 20 minutes before declaring death, particularly when organ donation is being considered....

But the criteria used to establish brain death have slight variations across the globe.

In Canada, for example, one doctor is needed to diagnose brain death; in the UK, two doctors are recommended; and in Spain three doctors are required. The number of neurological tests that have to be performed vary too, as does the time the body is observed before death is declared.