How will IRS notify me of declined return ?

[AnthonyC]

If you are referring to your initial acceptance or rejection - You will receive email notification of whether your return has been accepted or rejected (assuming you electronically filed).  If you are unsure if you got an email, you can check your e-file status at the link below.

https://turbotax.intuit.com/tax-tools/efile-status-lookup/

If you are referring to the actual processing of your return, the IRS will notify you via letter if there is a problem with your return (i.e they are disallowing a credit or deduction) and they will adjust your return.

You can check on your general refund status at the link below - 

https://www.irs.gov/refunds