How can I check which feature of the app are used and where?
AM Utils doesn't store the information about each instance where its function is used. For that you'd need to look at which workflows, post-functions, validators are used and in what way. We do have this feature in our backlog but it's not planned for any immediate sprints, and this could be several months for it to get implemented
Starting with v2.2.0 version of AM Utils and above you are able to find out how much the AM Utils app is used in your Jira instance.
To do this, go to Administration → Manage apps → Am Utils → Workflow Viewer. For more details, click here.