Hi,
When and where depends on the tested application and the way how scrollbars are implemented by the given programming language/library control.
If TestComplete's Object Spy cannot recognize the scrollbar as a separate object, the generic approach is to select the parent object (usually this is either grid, or edit box, or the similar control) and to examine its properties. Quite often there are properties like HScroll, VScroll, Scroller, etc. which provide more or less usable access to the scrollbar's properties and methods.