Two parts:

$ brew tap agamdua/homebrew-awsprey
$ brew install awsprey

Set AWS credentials with priveleges to describe EC2 instances. This currently uses your default AWS profile. Future versions will allow you to use custom profiles.

Check if you have a file ~/.aws/credentials that looks like:

[default]
aws_access_key_id = xxxxxxxxxxxxxx
aws_secret_access_key = xxxxxxxxxxxxxxx

If not, create one and add your values there.

Once you are done setting up, assuming your EC2 instances are tagged with service and environment tags:

$ awsprey list <service>:<environment>

If you have a service called web and want to check environment staging:

$ awsprey list web:staging