The Last Touched Date of a case can be found on your Dashboard under Inactive Cases, on the Home Tab of a case, Reports, and Management screens. These dates will automatically update when your firm works on a client's case. Any type of manual or autonote will modify the date. Simply clicking into a case will not update this date.
Keeping an eye on these dates is a great way to ensure cases don't slip through the cracks. Try sorting the My Cases or Open Cases Report by the Last Touched Date column header to view the most inactive cases.