If for any reason you are uncomfortable with how to choose investments in your 401k or how to diversify your 401k, read as much information on investing as you can.  Read the prospects of each investment chosen in your 401k.  Read the 401k information your employer puts out as well as any information on your 401k’s web site.  Another good source for information is to check the web sites for the large mutual fund companies.    Vanguard’s web site is very good and has loads of information.  Just realize that they are trying to get you to buy their funds.  If you still do not feel comfortable after reading all the information you can gather, then find a professional to help you.  
 
Look for a Certified Financial Planner who is a fee-based planner.  Some financial planners are paid by what products they sell.  They receive a load or commission on this.  Be sure to ask them how they get paid, as it can be as high as 5% of your money right off the bat.   Look for someone who has 5 to 10 years of experience and preferably someone you know and who has a good reputation.  If all you want is to have them review your 401k, tell them that is it.  They will want you to do a full plan but only do what you need.