Up to 85% of Social Security 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
To see the Social Security Benefits Calculation Worksheet in Turbo Tax Online version you would have to save your return with all the worksheets to your computer. Or if you are using the Desktop CD/Download Software you can switch to Forms Mode (click Forms in the upper right) and click on SS in the list on the right side.
The taxable amount of SS will be on 1040 line 6b. If you didn't get any taxable amount on 6b and you got a letter from the IRS saying it was wrong you might have answered a question wrong. There is a new question this year asking if you lived in a foreign place. People have been answering it wrong or skipping it. Go back through the Social Security entries and check, check for each spouse if married. If the IRS adjusted your return you do not need to amend.
