How to check that a JCheckBox is checked?

Use the isSelected method.

You can also use an ItemListener so you'll be notified when it's checked or unchecked.

By using itemStateChanged(ItemListener) you can track selecting and deselecting checkbox (and do whatever you want based on it):

myCheckBox.addItemListener(new ItemListener() {
    @Override
    public void itemStateChanged(ItemEvent e) {
        if(e.getStateChange() == ItemEvent.SELECTED) {//checkbox has been selected
            //do something...
        } else {//checkbox has been deselected
            //do something...
        };
    }
});

Java Swing itemStateChanged docu should help too. By using isSelected() method you can just test if actual is checkbox selected:

if(myCheckBox.isSelected()){_do_something_if_selected_}