Depending on how you have your event displayed on your website, when you hit the Event Capacity limit, the words SOLD OUT will appear in red text in the following formats:

If you are linking to the event description for an event with one performance date

Customers will be redirected to your public event listing with a notification that the event is sold out. Customers will have the opportunity to join the waitlist if it is enabled in the event settings.
 

If you are linking to a list of performances

Customers will see the Sold out message next to any/all performances that are sold out.

If you use the calendar display, the pop-up for a sold out event will appear as follows:

If you are linking directly to the ticket quantity selection page of one performance
Customers will see the Sold Out message right away.

To learn more about the waitlist, please see the following discussion:
How the waiting list works

To learn how to alter the Sold Out message, please see the following discussion:
Can I modify or hide the Sold Out notice?

For more information on the various ways to display your events, please see the following discussion:
Using Get Code Buttons

To get an email alert when your event sells out, please head to your Account Settings > Email Notifications.  Add your email to the alert for "When an event sells out".