Start by talking to your HR department at work. They should be able to point you in the right direction.
As Curt said, HR contact is the best place to start. If you have an old statement available, it will probably have an 800 number you can call also. Balances are usually available on a plan provider's website, but of course you would need your log-in info.
The plan administrator is required to send you a statement at least once a year. So if you have worked for the company more than a year you can call the plan administrator and get the information you need. If you have been there less than a year and have not received a statement, go to the benefits or personnel department to find out how to contact the plan administrator.
The information you should be looking for is not just the ending balance, but how the money in your 401k is invested and if you should be making any changes.