The NDW contains a Medicaid indicator on the registration record. The insurance category code (INSUR_CAT_CD field) will be MCD if they are eligible for Medicaid. There is also a Medicaid eligibility flag on the encounter record (MCD_ELIG_FG field). This may mean the person receives SSI, since that program is a subset of Medicaid. However, the NDW does not have a designated indicator for SSI.
The field INSURER_NM in the REG.INSUR_ELIG table is meant to hold the name of the insurance for a registrant. As a free-text field, the lack of structure in the names makes this field difficult for analysis, but on rare occasions it will hold values that indicate SSI (SOCIAL SECURITY ADMINISTRATION, SSI, SOCIAL SECURITY DISABILITY, etc.)