Open the invoice you want to view the history for, and use the menu on the top right corner to access the View History option

Each invoice has a record of it's history stored where you can see how and when it was generated, each time is has been emailed and each time it has been edited or had an update to the status. Notably, the user field will show whether the invoice has been system generated or generated by an individual user which will have implications such that user generated invoices will not automatically email.