Use GetBucketCors with an AWS SDK or CLI

        /// <summary>
        /// Retrieve the CORS configuration applied to the Amazon S3 bucket.
        /// </summary>
        /// <param name="client">The initialized Amazon S3 client object used
        /// to retrieve the CORS configuration.</param>
        /// <returns>The created CORS configuration object.</returns>
        private static async Task<CORSConfiguration> RetrieveCORSConfigurationAsync(AmazonS3Client client)
        {
            GetCORSConfigurationRequest request = new GetCORSConfigurationRequest()
            {
                BucketName = BucketName,
            };
            var response = await client.GetCORSConfigurationAsync(request);
            var configuration = response.Configuration;
            PrintCORSRules(configuration);
            return configuration;
        }

The following command retrieves the Cross-Origin Resource Sharing configuration for a bucket named amzn-s3-demo-bucket:

aws s3api get-bucket-cors --bucket amzn-s3-demo-bucket

Output:

{
    "CORSRules": [
        {
            "AllowedHeaders": [
                "*"
            ],
            "ExposeHeaders": [
                "x-amz-server-side-encryption"
            ],
            "AllowedMethods": [
                "PUT",
                "POST",
                "DELETE"
            ],
            "MaxAgeSeconds": 3000,
            "AllowedOrigins": [
                "http://www.example.com"
            ]
        },
        {
            "AllowedHeaders": [
                "Authorization"
            ],
            "MaxAgeSeconds": 3000,
            "AllowedMethods": [
                "GET"
            ],
            "AllowedOrigins": [
                "*"
            ]
        }
    ]
}

Get the CORS policy for the bucket.

import {
  GetBucketCorsCommand,
  S3Client,
  S3ServiceException,
} from "@aws-sdk/client-s3";

/**
 * Log the Cross-Origin Resource Sharing (CORS) configuration information
 * set for the bucket.
 * @param {{ bucketName: string }}
 */
export const main = async ({ bucketName }) => {
  const client = new S3Client({});
  const command = new GetBucketCorsCommand({
    Bucket: bucketName,
  });

  try {
    const { CORSRules } = await client.send(command);
    console.log(JSON.stringify(CORSRules));
    CORSRules.forEach((cr, i) => {
      console.log(
        `\nCORSRule ${i + 1}`,
        `\n${"-".repeat(10)}`,
        `\nAllowedHeaders: ${cr.AllowedHeaders}`,
        `\nAllowedMethods: ${cr.AllowedMethods}`,
        `\nAllowedOrigins: ${cr.AllowedOrigins}`,
        `\nExposeHeaders: ${cr.ExposeHeaders}`,
        `\nMaxAgeSeconds: ${cr.MaxAgeSeconds}`,
      );
    });
  } catch (caught) {
    if (
      caught instanceof S3ServiceException &&
      caught.name === "NoSuchBucket"
    ) {
      console.error(
        `Error from S3 while getting bucket CORS rules for ${bucketName}. The bucket doesn't exist.`,
      );
    } else if (caught instanceof S3ServiceException) {
      console.error(
        `Error from S3 while getting bucket CORS rules for ${bucketName}.  ${caught.name}: ${caught.message}`,
      );
    } else {
      throw caught;
    }
  }
};

class BucketWrapper:
    """Encapsulates S3 bucket actions."""

    def __init__(self, bucket):
        """
        :param bucket: A Boto3 Bucket resource. This is a high-level resource in Boto3
                       that wraps bucket actions in a class-like structure.
        """
        self.bucket = bucket
        self.name = bucket.name


    def get_cors(self):
        """
        Get the CORS rules for the bucket.

        :return The CORS rules for the specified bucket.
        """
        try:
            cors = self.bucket.Cors()
            logger.info(
                "Got CORS rules %s for bucket '%s'.", cors.cors_rules, self.bucket.name
            )
        except ClientError:
            logger.exception(("Couldn't get CORS for bucket %s.", self.bucket.name))
            raise
        else:
            return cors

require 'aws-sdk-s3'

# Wraps Amazon S3 bucket CORS configuration.
class BucketCorsWrapper
  attr_reader :bucket_cors

  # @param bucket_cors [Aws::S3::BucketCors] A bucket CORS object configured with an existing bucket.
  def initialize(bucket_cors)
    @bucket_cors = bucket_cors
  end

  # Gets the CORS configuration of a bucket.
  #
  # @return [Aws::S3::Type::GetBucketCorsOutput, nil] The current CORS configuration for the bucket.
  def cors
    @bucket_cors.data
  rescue Aws::Errors::ServiceError => e
    puts "Couldn't get CORS configuration for #{@bucket_cors.bucket.name}. Here's why: #{e.message}"
    nil
  end

end