Every 401(k) plan is required to have a plan administrator who takes care of all record keeping for the plan and a custodian who holds the assets. Many good plan custodians provide online access to your individual 401k account where you can usually check your daily balance and investment allocation. At a minimum, you should be receiving end of year statements with your total account value and allocation by investment. If this plan is with a current employer, I suggest you call your human resources department and ask who administers and acts as custodian for your plan. If it's an old employer 401k account, look for your year end statement and find the custodian's website on it or call you former employer's human resources and inquire with them.
The best place to start gathering information about your workplace benefits, including 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 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. Another option would be to call the custodian directly and request your 401k balance.