The tags of the ELB contain the information you're looking for.
$ aws elb describe-tags --load-balancer-names xxxxx
{
"TagDescriptions": [
{
"LoadBalancerName": "xxxxx",
"Tags": [
{
"Value": "default/nginx",
"Key": "kubernetes.io/service-name"
},
{
"Value": "my-cluster",
"Key": "KubernetesCluster"
}
]
}
]
}