The best place to start start gathering information about your 401k, is with your company's human resources department. They will be able to tell you which custodian is managing the retirement plan. The custodian should be reporting your balance on monthly or quarterly statements (whether electronically or via paper). They may also have an option for you to create an online account and check your balance whenever you want. If you don't have online access and are not receiving statements, your best option would be to call the custodian directly and request your 401k balance.
Perhaps this article is helpful. http://www.brightscope.com/financial-planning/advice/article/24529/10-Ways-To-Find-An-Old-401K-Plan/