This assumes you have already configured the aws command line tool. If not, check out my article on Getting Started with the AWS CLI.

The aws ecs list-clusters command can be used to list your Elastic Container Service (ECS) Clusters.

~]$ aws ecs list-clusters
{
    "clusterArns": [
        "arn:aws:ecs:us-east-1:123456789012:cluster/my-ecs-cluster"
    ]
}