I suggest that you make an appointment with your company's HR department. They should be able to provide you, if available, online access to your account or at the very least, look your balance up for you.
If your company has a small 401(K), it might not provide access to a national service. Ther easiest thing to do is ask for performance figures from the account representative. Then you can take the information and go to a site like www.yahoo.com - finance. Good luck.
The easiest way is to call Human Resources and ask for instructions to the online portal. If they cannot quickly provide this to you, get the phone number of the plan administrator who will provide it to you. You can access the details of your account there. You can also ask for a ‘Summary Plan Description’.