The following command lists all of the active multipart uploads for a bucket named my-bucket:

aws s3api list-multipart-uploads --bucket my-bucket

Output:

{
    "Uploads": [
        {
            "Initiator": {
                "DisplayName": "username",
                "ID": "arn:aws:iam::0123456789012:user/username"
            },
            "Initiated": "2015-06-02T18:01:30.000Z",
            "UploadId": "dfRtDYU0WWCCcH43C3WFbkRONycyCpTJJvxu2i5GYkZljF.Yxwh6XG7WfS2vC4to6HiV6Yjlx.cph0gtNBtJ8P3URCSbB7rjxI5iEwVDmgaXZOGgkk5nVTW16HOQ5l0R",
            "StorageClass": "STANDARD",
            "Key": "multipart/01",
            "Owner": {
                "DisplayName": "aws-account-name",
                "ID": "100719349fc3b6dcd7c820a124bf7aecd408092c3d7b51b38494939801fc248b"
            }
        }
    ],
    "CommonPrefixes": []
}

In progress multipart uploads incur storage costs in Amazon S3. Complete or abort an active multipart upload to remove its parts from your account.