How can I find out which button was clicked?

In order to get label, try this.

ActionListener actionListener = new ActionListener()
{
      public void actionPerformed(ActionEvent actionEvent) {
            JButton button = (JButton)actionEvent.getSource();
            String label = button.getLabel(); //Deprecated 

            String label2 = button.getText();
     }
};

ActionEvent has a method getActionCommand() that will get a JButton's actionCommand String. This is usually it's text as well (for JButtons).

try this

ActionListener actionListener = new ActionListener() {
    public void actionPerformed(ActionEvent actionEvent) {
        System.out.println(actionEvent.getActionCommand());
    }
};