If you're talking about an account that gets funded by paycheck deductions, like a 401k, 403b, or 457, then you should be receiving quarterly or at least annual statements. If you are not receiving them, they may not have your current address and you should contact your HR department. If you have an old statement, it should have the contact information on it. You can call to find out your balance.
Contact your HR department for contact information of your plan's administrator. They'll give you a toll free customer service line or a website that will get you the information you seek.