How do I determine if my mother's SS benefits are taxable? I claim her as a dependent and am trying to understand if her SS benefits is income

Unless she has other types of income then none of the SS benefits are taxable.

Up to 85% of Social Security Retirement/Disability/Survivors benefits becomes taxable when all your other income plus 1/2 your social security reaches:

Married Filing Jointly - $32,000
Single or Head of Household - $25,000
Married Filing Separately - 0

Did she have any other income of any type in 2020?  If not, then none of it is taxable.

But in determining whether you provided more than 50% of her Support, you look at her gross (not taxable) income: the $10,000 plus any anything else she received (such a subsidized housing, food stamps, Medicaid, etc.