if it's not clear from the IRS notice an approach would be to review your tax return transcript. if it's available it shows details of what you submitted and what the IRS computers show. this may enable you to determine if the IRS is correct.
https://sa.www4.irs.gov/eauth/pub/es_icce.jsp
another approach is to use
The Audit Defense service is provided by TaxResources, Inc., also called TaxAudit.com, in partnership with TurboTax. If you paid for Audit Defense and you received an IRS notice, call TaxResources, Inc. at 877-829-9695, or report your IRS notice on their website at http://intuit.taxaudit.com/. Do not contact the IRS until you have spoken to TaxResources. They may contact the IRS on your behalf.
or you can hire an independent tax pro - CPA or tax lawyer.
do not delay. that notice probably gives you only so many days to respond. failure makes it much more difficult to fight the issue even if the IRS is wrong.
