You can use this to connect your window to the show signal:
g_signal_connect (G_OBJECT (window), "show", G_CALLBACK (window_is_visible), (gpointer) data);
The first parameter is the widget that you want to monitor for the "show" signal. When the window is shown to the user, window_is_visible() will be called. The fourth parameter allows you to send one pointer to the callback function, or NULL to ignore it. The callback function should look like this:
void window_is_visible (GtkWindow *window, gpointer data)
/* Do something now that the window is shown. */
Hope this helps!