The easiest and fastest way to find out if the Windows taskbar's auto hide feature is on is to use the Windows SHAppBarMessage() function. Here's the Object Pascal code port:
uses
ShellAPI;
function TaskbarAutoHide : boolean;
var
ABData: TAppBarData;
begin { TaskbarAutoHide }
ABData.cbSize := SizeOf(ABData);
Result := ((SHAppBarMessage(ABM_GETSTATE, ABData) and ABS_AUTOHIDE)>0)
end; { TaskbarAutoHide }