The Qt Creator it is possible to select from right click menu on a QPushButton, QAction etc.

the entry "Slot anzeigen..." (german), in english something like "Show slots...".
Don't know the english entry, using only the german language for Qt Creator.

This following dialog will be shown:

It would be very helpfull, if this dialog shows the already created slots, e.g. with
an icon as prefix  or "exist" in a new column.

The style could be look like "Insert virtual function". There they are checked and grayed out.