Your best resource is your current plan administrator. Make an appointment with them to discuss the most recent 408(b)(2) and 404(a)(5). These pertain to the Department of Labor's Fee Disclosure Rules. Your Plan Administrator should be able to walk you through these disclosures and help you understand the inner workings of your 401k plan.
In addition to the excellent advice above, you need to determine the individual fund fees. Many times the largest fees in your plan are the individual fund fees. If you can get the symbols of the funds offered you can go to http://www.finra.org/investors/tools. This site is run by FINRA and you can calculate these charges and compare them against lower cost options.
As an additional means of getting this information your employer has a responsibility to educate the participants of the plan about these types of issues so that you can make informed decisions on the investments you select. They should have materials readily available for you on all of the questions pertaining to fee's that you might have. If there is a person at your employer or previous employer who is responsible for the plan that person should easily be able to point you int he right direction.
Each year you're required to get a participant fee disclosure for your account (the 404(a)(5) thingy the other answer referenced). It outlines the fees for your account, certain types of transactions (taking a loan, etc) as well as how much you're spending on investment management fees (quoted as a percentage and a dollar amount per $1000). That in combination with your statement is your best resource.