How do I determine if a tax filing error was a turbotax error or user error?

It is usually user entry error.  What is wrong?

Just a guess.......did you claim excess Social Security?  That's a common user error if you are married.

We do not have enough information from you so all we could do is guess.  Did you receive a letter from the IRS or your state?  TurboTax does not get any information from the IRS or your state after your return is filed, so no one at TT knows what they changed or challenged on your return.  As VolvoGirl said, most errors are user errors due to either missing information that you did not enter, or information entered incorrectly in some way--either by typos in data entries or by entering data in the wrong place or under the wrong name on a joint return.

(what the letters from the IRS mean)

I GOT A LETTER FROM THE IRS   

https://ttlc.intuit.com/questions/3605673-i-got-a-letter-or-notice-from-the-irs

If you tell us more we may be able to help you figure out what happened.