Regardless if you are starting out with frozen or precooked frozen chicken cordon bleu, it should be heated to an internal temperature of 165°F (74°C). This is easy to check with an instant-read thermometer inserted into the middle of the chicken. If it is almost there, you can wrap it in aluminum foil for a few minutes and the temperature will rise a few degrees. If you don't have a meat thermometer you can cut into the chicken to check. If cooking raw chicken cordon bleu, make sure the chicken is white through and the juices run clear. If cooking precooked chicken cordon bleu, make sure it is thoroughly heated.