Unreconciled Account Periods are shown on the Engagement Dashboard as a black bar, as opposed to the reconciled green shade:
Hovering over an account period will create a pop-up window showing the start and end date for the period as well as the status (reconciled or unreconciled).
Clicking on a unreconciled account period will take you to the Statement Summary screen where the errors can be fixed and the account period then placed into a reconciled mode.