There should be a small sticker inside the fuel door that indicates that E85 is allowed.
Since yours is a 2004 and is the L59 engine (based on the "Z" in the VIN) it's Flex, and the fuel cap doesn't HAVE to be yellow. Here's part of my '04's build sheet from GM:
Since yours is a 2004 and is the L59 engine (based on the "Z" in the VIN) it's Flex, and the fuel cap doesn't HAVE to be yellow. Here's part of my '04's build sheet from GM: