Simply click on the Sessions link in the top menu, right next to Questions.
You'll now see a list of all live and ended Sessions. A blue dot means that the Session is still live, so new students can join with the given code. If it is a grey dot, this Session has already been ended and cannot be joined anymore.
Pro Tip: Keep your Session list clean by hovering with your mouse over the specific Session and then clicking on the "trash can" icon that will appear on the right.
Note: You can end a Session inside the Dashboard. This action is not reversible; students will not be able to join anymore.