When you check-in to a venue via the ‘+’ in your tab bar, the check-in will create a newsflash on your newsfeed. You will see a button that says ‘Attending’.
To check-out of a venue tap ‘Attending’ and follow the instructions to check-out.
If you want to check-out via Explore, you will see ‘Attending’ next to other peoples names if you’re also checked into that event. Tap ‘Attending’ and follow the instructions to check-out.