Ask HN: How do you keep an inventory of cloud infrastucture

I have inherited about a dozen AWS accounts to administer, and want to get a lay of the land of what is running on them. Basically looking for a tool where you can set up rules that give a one sentence description of each resource, and is also good at alerting once something new runs. From what I've seen, a significant percentage of EC2 instances are servers that can probably be terminated because their purpose was to run some random tests 2 years ago. I understand that ideally everything would probably be part of a CloudFormation template, but that is not a easy change to do once you already have things running AFAIK.