Which actions are you talking about? Repository-tier actions or Share client / doclib actions? Depending on this, different web scripts may apply (or a single, unified listing may not even be available), and the way to look for implementations may be different.
The endpoint <host>:<port>/alfresco/s/api/actiondefinitions provides a list of Repository-tier actions using the ActionService framework.
There may of course be other actions there implemented by custom ReST endpoints, but since these do not follow any common pattern or adhere to a specific API (apart from web scripts), they are impossible to be listed.