You need to first find out who the custodian and/or administrator for the plan is. You should be receiving statements from the custodian at least every quarter. In most cases you should be able to set up online access to your account and/or received statements electronically. If you are unsure who the custodian is, check with your human resources department, and find out who the plan administrator is.
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.