Java: How to find if a method is overridden from base class?

How to find out if a method is overridden by child classes?

For example,

public class Test {

static public class B {
    public String m() {return "From B";};
}

static public class B1 extends B {

}

static public class B2 extends B {
    public String m() {return "from B2";};
}

/**
 * @param args
 * @throws FileNotFoundException 
 */
public static void main(String[] args)  {

    B b1 = new B1();
    System.out.println("b1 = " + b1.m());
    B b2 = new B2();
    System.out.println("b1 = " + b2.m());
}

}

Given an instance of B, how do I know if any derived classes have overridden method m() like B2?

Update: My question wasn't clear. Actually, I was trying to ask if this is possible without resorting to reflection. This check is done in a tight loop and it's used in a performance hack to save a few CPU cycles.