All of the information Mr. Patton answered is correct. I would only add that if your mother is deceased, another idea for you is to check the National Association of Unclaimed Property Administrators. Start your free search for money that might be due you by going to http://www.unclaimed.org.
It would depend on the circumstances. If your mother is still living then you would not be able to get any information unless you had a power of attorney and jumped through some hoops with the service provider of the 401k. If your mother is unfortunately deceased a call to the service provider will allow them to give you the information they will need to disperse the funds to the beneficiaries. Most likely you will need an official copy of the death certificate for any of this. To find out who is the service provider you can either contact the HR department of the company your monther worked for. Hope this helps.