How to track the inactive assets in AWS

While the question is valid, there is no simple answer for it. In the end it will be a bit different methods and tools for each resource and environment. You could also ask "How do I track/manage the lifecycle of all my applications/deployments?". This might take you closer to root cause of why inactive resources are left behind. If you are interested about the topic you might want to check finops.org and specially the waste management topic https://www.finops.org/projects/reducing-waste/