The UIManager.getSystemLookAndFeelClassName() method returns the current system LAF (Look and Feel) for Swing application. Do not forget to call the SwingUtilities.updateComponentTreeUI() method after setting the LAF to update the current application look and feel.

package org.kodejava.swing;

import javax.swing.JFrame;
import javax.swing.SwingUtilities;
import javax.swing.UIManager;
import javax.swing.WindowConstants;

public class SystemLAFDemo {
    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setSize(500, 500);
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.setTitle("System LAF Demo");

        try {
            // Use the system look and feel for the swing application
            String className = UIManager.getSystemLookAndFeelClassName();
            System.out.println("className = " + className);
            UIManager.setLookAndFeel(className);
        } catch (Exception e) {
            e.printStackTrace();
        }

        SwingUtilities.updateComponentTreeUI(frame);
        frame.setVisible(true);
    }
}

System Look and Feel in Swing