Verify a parent window?

I'm getting a list of all the visible windows on the desktop and want to find out by using the window's handle if it is a parent window. I know about the GetParent API:

GetParent returns the handle of the parent window of another window. For example, the parent of a button would normally be the form window it is in. If successful, the function returns a handle to the parent window. If it fails (for example, trying to find the parent of a non-window), it returns 0.

When I try this code for a known parent window it returns 0.
But I'm not sure if its correct to assume that if it returns 0 that it is a parent window.

Is there another way to do this, or is my assumption correct?

If it returns 0, it means that the window is already a top-level window (i.e. it has no parent).