Nowadays, with the explosion of information technology and internet, you can easily sit at home or at the old company to research about the employer on its website or by asking friends and others (those who know about that company.
Do not start writing the resume until you have any idea about the employer. Also, you should remember this important point: you need to show the employers what you can benefit them but not what you may benefit from them. The perfect resume must focus on the strength in necessary experience and skills that the employer may require from you. You will score more point with your knowledge about the employers and understanding of what they expect from you.