Everything you do when working with an application - from status changes to shortlisting to sending emails - is stored on the application history.
To access this audit trail, first visit the application list.
Click on the name of the applicant to see their application summary.
Next click the History button at the top of the summary page.
This will show you the date & time all actions carried out with this application. The text of emails sent to the applicant can be read by clicking View Email.
Head back to the Application List by clicking Applications on the vacancy header.