应该可以使用以下测试来检查非utf：

MY_STRING IS NOT NULL AND TRY_HEX_DECODE_STRING(HEX_ENCODE(MY_STRING)) IS NULL

但是那时我没有数据可以测试。
要将字符串重新编码为utf-8，可以使用JavaScript函数：

CREATE OR REPLACE FUNCTION TO_UTF8(BINARY_TEXT BINARY)
RETURNS TEXT LANGUAGE JAVASCRIPT STRICT IMMUTABLE AS '
  var win1252 = [ /* C1 controls */
    8364,  129, 8218,  402, 8222, 8230, 8224, 8225,
     710, 8240,  352, 8249,  338,  141,  381,  143,
     144, 8216, 8217, 8220, 8221, 8226, 8211, 8212,
     732, 8482,  353, 8250,  339,  157,  382,  376
  ];
  return String.fromCharCode(
    ...Array.from(BINARY_TEXT).map(x => (x < 128 || x > 159) ? x : (win1252[x - 128]))
  ); /* .map(...) can be removed if no conversion from win1252 needed */
';

SELECT NVL(TRY_HEX_DECODE_STRING(HEX_ENCODE(MY_STRING)),
           TO_UTF8(HEX_ENCODE(MY_STRING)::BINARY));