When entering a waved event, you will be asked to select your preferred wave(s).
There is a good chance you'll be put into your first or second choice. However, your wave will not be confirmed until 10 days before race day, when you will receive an email confirmation.
If you would like to know which wave you're currently in, you can log into your Racer Dashboard and check in 'My Entries'. The wave currently assigned will show under the ticket in the My Entries tab. If you think there has been a mistake and you've been placed in the wrong wave, please contact the event organiser. See the article on how to do this here.
Note: If you're racing as part of a team and you are the Team Captain, your team will all be placed in the same wave as you. However, this may not always be the case if you're doing different distances.