Determine if scrollbar's current position is beyond maximal/minimal value

case WM_MOUSEWHEEL:
    {
        // determine if treeview control has visible vertical scrollbar
        LONG_PTR WindowStyle = GetWindowLongPtr( hwnd, GWL_STYLE );

        if( WindowStyle & WS_VSCROLL )
        {
            SCROLLINFO si;

            ZeroMemory(&si, sizeof(si));
            si.cbSize = sizeof(SCROLLINFO);
            si.fMask = SIF_ALL;

            GetScrollInfo( hwnd, SB_VERT, &si );

            // here is the problem: I do not know how to formulate the condition
            if( /* currentPosition > Min or currentPosition < Max value */ )
                return 0L;   // handle it by doing nothing

            // otherwise do our stuff
        }
    }
    return 0L; // if scrollbar is not visible handle it by doing nothing