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"
    ]
}

And then the aws ecs list-container-instances command can be used to list the EC2 Instances that have been registered in the cluster. In this example, there is one EC2 Instance registered in the ECS cluster.

~]$ aws ecs list-container-instances --cluster arn:aws:ecs:us-east-1:123456789012:cluster/my-ecs-cluster
{
    "containerInstanceArns": [
        "arn:aws:ecs:us-east-1:123456789012:container-instance/my-ecs-cluster/296c9008473948f89c8b2398c914bf0b"
    ]
}