how can I learn this before connecting anything.

example

```
does_signal_exist(widget, signal):
    pass

button = Gtk.Button()

does_signal_exist(button, "clicked") # returns True
does_signal_exist(button, "hello") # returns False
does_signal_exist(button, "unboxed") # returns False
does_signal_exist(button, "button-press-event") # returns True

```