It all depends on what type of GtkContainer you are working with. You can use gtk_container_get_children() to get all of the container's children as a doubly-linked list.
May be I misunderstood what you said. If I could use gtk_container_get_children then I already have known that gtkcontainer and I did not need to find it.
I'm looking for the inverse problem, say,
a) I have a gtkentry and gtkfixed
b) use gtk_fixed_put to add gtkentry to gtkfixed
c) later, starting from the gtkentry, I would like to find which gtkfixed it is added.