How can I know if my family is qualified for the back-door IRA and how to make that happen?

The banker which is helping to set up the back-door IRA asked us to check with the accountants that to review our Form 1040  to understand if we are qualified to set up the back-door IRA. 

 

Which number(line) and place where we should look from Form 1040 to know if we are qualified for the setup? What is the threshold for the back-door IRA to allow setting up?