You can use the same methods used to book them into a class to mark your member's attendance after an event has passed. 

First, head to your calendar and select the event, then click "View". 

If your member has been booked into the event (This article explains the ways that you can book members into events from the Calendar) you can mark their attendance by clicking on the three dots and selecting "Check in" from the dropdown list to mark as attended or "Mark absent".

First, you will need to go to the member's profile > Attendance tab. As long as the member has been booked into the event (This article explains the ways that you can book members into events from their profile) you can mark their attendance by clicking on "Select" and then either "Attended" or "Absent".

First, open class kiosk mode from the Dashboard tab. 

From here members can 'Drop In' to an event if they have not booked into the event, or you can mark their attendance by selecting the 'Check In' button if so. 

See this article for more information on the Class Kiosk mode.