To activate the RadBusyIndicator control you have to set its IsBusy boolean property to True.
[XAML] Setting the IsBusy property
You can data bind this property in any way that suits your custom logic. The RadBusyIndicator control will be visible only when the IsBusy property is set to True.
RadBusyIndicator also gives you the IsBusyIndicationVisible property. It is a read only boolean property which value will be equal to True whenever the RadBusyIndicator is enabled and the busy content is still visualized.
To read more about the possible scenarios where you can use the IsBusyIndicationVisible property, look here.