The method isKeyboardTrigger of the class ContextMenuEvent returns false also for keyboard triggered context menus.

import javafx.embed.swt.FXCanvas;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import org.eclipse.swt.SWT;
import org.eclipse.swt.layout.FillLayout;
import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.widgets.Shell;

public class TwoButtons {

  public static void main(String[] args) {
    final Display display = new Display();
    final Shell shell = new Shell(display);
    shell.setLayout(new FillLayout(SWT.VERTICAL));

    final FXCanvas fxCanvas = new FXCanvas(shell, SWT.NONE);
    Button button = new Button("Right click or press menu key!");
    Scene scene = new Scene(button);
    fxCanvas.setScene(scene);

    button.setOnContextMenuRequested(event ->
        button.setText("trigger is: " + (event.isKeyboardTrigger() ? "keyboard" : "mouse")));

    shell.open();
    while (!shell.isDisposed()) {
      if (!display.readAndDispatch()) {
        display.sleep();
      }
    }
    display.dispose();
  }
}