You can also configure it using aws configure or if already executed you can identify it in ~/.aws/config. Example:
[default] region=us-east-1
The regions are as follows. See second column.
$ ec2-describe-regions REGION eu-central-1 ec2.eu-central-1.amazonaws.com REGION sa-east-1 ec2.sa-east-1.amazonaws.com REGION ap-northeast-1 ec2.ap-northeast-1.amazonaws.com REGION eu-west-1 ec2.eu-west-1.amazonaws.com REGION us-east-1 ec2.us-east-1.amazonaws.com REGION us-west-1 ec2.us-west-1.amazonaws.com REGION us-west-2 ec2.us-west-2.amazonaws.com REGION ap-southeast-2 ec2.ap-southeast-2.amazonaws.com REGION ap-southeast-1 ec2.ap-southeast-1.amazonaws.com