When a ticket is created in your portal, you could set a priority to the ticket according to the subject and level of urgency expressed by the customer. In the SLA policy, you would be able to set a first response time as well as resolution time. Please navigate to Admin -> Workflows -> SLA policies -> click on edit next to the policy. 

When the first response time is violated the ticket would contain a "response due" tag which could be seen when you see this ticket in the queue within your tickets list view. 

The resolution time when violated would give the ticket a tag called "overdue" which can also be seen when you check your queue.