How to show the validation tooltip without hovering the red indicator in cell?

You can show the validation tooltip in SfDataGrid without hovering the red indicator. For this, you need to write style for GridCell and set the ValidationToolTip.IsOpen property as true HasError visual state and false in NoError visual state.

XAML:               

Note: This workaround only recommended when you show the validation errors using CurrentCellValidating or RowValidating events. If you set this when built-in validation (using DataErrorInfo, INotifyDataErrorInfo and Data Annotations) enabled, then tooltip will be displayed in all the cells which has error.

Sample:

WinRT

UWP