You can either talk with your HR department to get the name of you companies provider or if you know, just call your provider, Fidelity, John Hancock, American Funds. They ask for your SSN to verify identity. Once they've verified who you are, they'll provide you with your curretn account information. Of course, if you have an old account statement, you can also just go on-line to get updated values.
The Benefits Administrator for your company can usually provide you with a website or telephone number to the firm that handles your 401K account. Ideally, you would be receiving statements annually, semi-annually or quarterly.
Many who leave their employer do not realize that they may transition their entire 401K into an IRA ROLLOVER Account (by following a few steps with your Benefits Administrator). By doing so, you have more control over these assets and an RIA (like myself) can assist you in determining how to invest wisely based on your needs and time horizon.
Your plan sponsor should be reporting your account balance to you on a quarterly basis by providing a statement or some other accounting record. If you are not receiving a statement this could indicate you are not a participant in the plan yet or the sponsor may simply have a bad address on file. Either way the thing to do is to contact your plan sponsor. Ask to speak to the Human Resources department for help.
Hope this helps. Have a nice weekend.