If it is a current 401k, you can go to your HR department or if with a small company, ask your boss. If from a former employer, call them
Try logging onto your company's retirement plan website and see if you can bring up the most recent statement or details; often this data is shown on such documents. Best of luck.
Try the 401k record keeper as they should be able to give you all the details you need.
Best of luck!
Perhaps this article is helpful. http://www.brightscope.com/financial-planning/advice/article/24529/10-Ways-To-Find-An-Old-401K-Plan/