Invoice History and User Timeline
There are a couple of methods for viewing the history of user's actions on sales records.
You can access this by opening any invoice, and clicking the Show History button located at the bottom left. This will show when the invoice was created, modified, sent, signed and so on, and by whom.
You can click on any entry that is highlighted blue to show a basic snapshot of the invoice at the time before the changes were made.
You can access this by going to Setup > Preferences > Users, then clicking on the Timeline clock icon located to the far right on the users list.
Once there, this screen will list any changes that User has made to invoices and other parts of the program, however, if you need more detail you may have to look at the Invoice History.