It's not easy to find, but there is a list of signals in the documentation. The available signals can be found under the
Public Attributes section. There you can unfold the inherited properties to get a full list of all signals supported by the widget.
In TGUI 0.8.6 I also added a Signals namespace that can be used as alternative to using the strings. You can now use tgui::Signals::EditBox::ReturnKeyPressed instead of "ReturnKeyPressed", which means that auto-complete in an IDE might also give you a list of possible signals. The
documentation of these structs also provide a list. I don't think the Signals namespace is mentioned in any tutorials yet, so it also easy to miss.
I always wanted to have a good overview of widgets somewhere, some place where it shows how every widget looks and what signals you can use etc, but I just don't have the time for it.