How do we know whether my mom's social security payments are taxable or nontaxable? She's 82 yrs old and doesn't have any income from work or rent or anything.

The Social Security Administration mails the benefits statement, Form SSA-1099, every January to the beneficiaries address on file with the SSA.

If the Social Security benefits are the Only income to be reported on the tax return, then there is no reason or need to file a tax return.

If Social Security is your mother's ONLY income she does not need to file a tax return.  Every year in January/early February she must be receiving a SSA1099 from Social Security that shows all the information regarding the SS benefits she has received, the deductions for Medicare, etc.  They mail it in January.

If she is on SS or SSDI she gets a SSA-1099 at the end of the year instead of a W2 or 1099R.  If she only gets SS or SSDI or ssi it is not taxable and she doesn't have to file a return.