how to check which constructor does the user use

I have my constructor like this

    public Name(String firstName, String lastName)
{
    setFirstName(firstName);
    setLastName(lastName);        
}

public Name(String firstName, String lastName, String middleName)
{
    setFirstName(firstName);
    setLastName(lastName);
    setMiddleName(middleName);
}

And I want to override my toString() method like this

    @Override
    public String toString()
    {
        if(no middle name)
        {return  getFirstName() + " " + getLastName();}
        else if(has a middle name)
        {return  getFirstName() + " " + getMiddleName() + " " + getLastName();}
    }

What should I do?