You can see a full history of everything that has happened on the lead via the Lead History popup. The Lead History button can be viewed from the calendar icon in the grey action menu via the 'History' button. This popup displays whenever a user opens a lead to view, when it was first viewed, when progress was updated, emails sent and more. The popup allows users to view the User that modified the record, their job title and the date/time the action was taken.
If you need more in-depth information on how a lead has been modified including tracking all changes to customer records, you can see this via the Audit button. The Audit button will be visible under the 'MORE' option in the grey action menu for any user with the Audit Role enabled.