How to know if I should file taxes for my mother's social security income?

I couldn't find any previous posts, so I'm relying on Glad's good research that your mother has a trust.

Not to be offensive or insulting, but if you're uncertain whether or not to report SS income, attempting to manage a trust return is going to be very, very challenging and frustrating.   It's not for the "faint of heart".   The first time around gave  me headaches; the second time around is different, and even harder b/c of different situations.

Is this  a Living Trust that's producing income that's being distributed?  If so, you definitely need a trust accountant.   If it's a Living Trust that merely holds assets for distribution after death, that's a different story, as long as there are no distributions. 

Post back with information on the Trust and we can offer more concise suggestions, including the basis method of determining whether SS is taxable.

Since she has a trust, you or your sister need to speak to a CPA. There may be, at the very least, forms for the trust that have to be filed.

Probably not if this is her only income. You can certainly call the IRS. Expect a bit of a wait time.

Not usually if it is her only income. Call AARP and ask.

Just checked your profile, you had asked another question regarding a trust that was setup for mom. That changes things. And sis it seems is mom's POA. It is her job to deal with it.