How can stop Elastic IP address not attached to a running instance per hour (prorated)??

In the EC2 console, under "Elastic IPs" you can check if the EIP has an "Association ID". If not then it isn't in use and should be deleted - you get charged for unused EIPs. That's what would appear on your bill. If it does have an Association ID you can also see the associated instance ID or NAT Gateway etc.

Still can't find it? Make sure you're looking at the right region. Cost Explorer can help narrow things down.