What you're looking for is IUIAutomation::ElementFromPoint method.

Here is a small console app (C++ with Visual Studio ATL) that continuously displays the name and window handle (if any) of the automation element under the cursor:

// needs
//#include <UIAutomationCore.h>
//#include <UIAutomationClient.h>

int main()
{
    if (SUCCEEDED(CoInitialize(NULL)))
    {
        CComPtr<IUIAutomation> automation;
        if (SUCCEEDED(automation.CoCreateInstance(CLSID_CUIAutomation8))) // or CLSID_CUIAutomation
        {
            do
            {
                POINT pos;
                if (GetCursorPos(&pos))
                {
                    CComPtr<IUIAutomationElement> element;
                    if (SUCCEEDED(automation->ElementFromPoint(pos, &element)))
                    {
                        CComBSTR name;
                        element->get_CurrentName(&name);
                        wprintf(L"name: %s\n", name);

                        UIA_HWND hwnd;
                        element->get_CurrentNativeWindowHandle(&hwnd);
                        wprintf(L"hwnd: %p\n", hwnd);
                    }
                }
                Sleep(500);
            } while (TRUE);
        }
    }

    CoUninitialize();
    return 0;
}