It appears that the Public Use File begins reporting the ASEC Income Supplement interview status code starting in 1991. The 1989-1991 ASEC technical documentation specifically notes that this variable (FL-665) is only available in 1991. I was unable to find a comparable variable in the 1988 documentation; you can find the codebooks on the IPUMS CPS documentation page.
You might take a look at the data quality flag variables as an alternative approach. These are typically linked in the variables FLAGS tab (see screenshot below for example). While pre-1991 samples include imputation flags for some individual ASEC income variables (e.g., QINCWAGE), these do not appear to identify whether full record imputation occurred.
