John did a great job of summing up how to go about getting that information. If you want a more specific answer you would need to provide more specific information -- Which I wouldn't recommend doing over a public forum such as this.
Contact the Human Resources Department of the employer who provides the 401(k) plan. You should be receiving statements on at least a quarterly basis. Often times when participants do not receive statements the reason is that the recordkeeper has the wrong mailing address in their system or they have set you up to receive electronic statements. In the case of electronic statements you should receive an email when the statement is ready for viewing. It could be they do not have a good email address for you. Either way the thing to do is to update your contact information at the Plan's recordkeeper. That way you will get a statement with your balance every calendar quarter in the future. Again, your employer's HR department can straighten this out for you.
Hope this helps.