It's rather premature for the IRS to be questioning whether or not a 2015 distribution was rolled over since they know that the rollover of a late-in-the-year distribution might not be completed until the beginning of the following year.
You'll need to use your account statements to substantiate the rollover. The 2016 Form 5498 reporting receipt of the rollover contribution will not be issued by the IRA custodian until May of 2017 (which is why it's premature for the IRS to be questioning this).
