When locking a bike you simply roll it into the dock. In order to check if the bike is in fact registered as delivered, see to that the bike:
Plays a little jingle
The light in front of the bike will blink green
The light in the back of the bike will stop blinking red
Most important: The app will tell you that "the bike has been returned", and show you the name of the bike, the station you biked to and from and how long the trip lasted.