Sub CheckNull()
    Dim cat As New ADOX.Catalog
    Dim tbl As New ADOX.Table
    Dim col As New ADOX.Column
    
    cat.ActiveConnection = "Provider=Microsoft.Jet.OLEDB.4.0;" & _
        "Data Source=D:\My Databases\dbTest.mdb"
    
    Set tbl = cat.Tables("Table1")
    Set col = tbl.Columns("FirstName")
    
    For Each col In tbl.Columns
        Debug.Print col.Name
        Debug.Print col.Properties("Nullable")
        'Debug.Print col.Properties("Jet OLEDB:Allow Zero Length")
        'Debug.Print col.Attributes
    Next
    
    Set cat.ActiveConnection = Nothing
    Set col = Nothing
    Set tbl = Nothing
    Set cat = Nothing
End Sub

Public Sub Main()
    Dim cnn As ADODB.Connection
    Dim strCnn As String
    Dim rst As ADODB.Recordset
    Dim strTable As String
    Dim adoField As ADODB.Field
    Dim adoProp As ADODB.Property
    
    strCnn = "Provider=Microsoft.Jet.OLEDB.4.0;" & _
        "Data Source=D:\My Databases\dbTest.mdb"
    Set cnn = New ADODB.Connection
    cnn.Open strCnn
   
    Set rst = New ADODB.Recordset
    strTable = "Table1"
    rst.Open strTable, cnn, adOpenForwardOnly, adLockReadOnly, adCmdTable
    
    ' Display the attributes of the connection
    Debug.Print "Connection attributes = " & cnn.Attributes
    
    ' Display the property attributes of the Employee Table
    Debug.Print "Property attributes:"
    For Each adoProp In rst.Properties
        Debug.Print "   " & adoProp.Name & " = " & adoProp.Attributes
    Next adoProp
    
    ' Display the field attributes of the Employee Table
    Debug.Print "Field attributes:"
    For Each adoField In rst.Fields
       Debug.Print "   " & adoField.Name & " = " & adoField.Attributes
    Next adoField

    ' Display fields of the Employee Table which are NULLABLE
    ' Это то, что тебе нужно.
    Debug.Print "NULLABLE Fields:"
    For Each adoField In rst.Fields
        If CBool(adoField.Attributes And adFldIsNullable) Then
            Debug.Print "   " & adoField.Name
        End If
    Next adoField

    rst.Close
    cnn.Close

    Set rst = Nothing
    Set cnn = Nothing
End Sub