Sorry for your loss...
Please see for reference the IRS publication 559 - http://www.irs.gov/pub/irs-pdf/p559.pdf
All income your mother received while alive is includible on the final return.
The word "DECEASED," the decedent's name, and the date of death should be written across the top of the tax return.
If a personal representative has been appointed, that person must sign the return.
If no personal representative has been appointed and if there is no surviving spouse, the person in charge of the decedent's property must file and sign the return as "personal representative"
You must attach Form 1310 to all returns and claims for refund. - http://www.irs.gov/pub/irs-pdf/f1310.pdf
All income received after your mother died is considered as income of her estate.
You need to collect all tax reporting forms W2, 1099's etc.
If you do not have these forms and do not have information about her income - you would not able to file her tax return.
You would need to contact her employer and other payers and request these forms.