I’m trying to book online and am unsure if the transaction has worked.

If your transaction has been successful you will be sent a confirmation email. Simply show the confirmation email on your smart phone and bring this along with you.
If you have not received a confirmation email it is unlikely your transaction has been successful. A reserve may be placed on funds by your bank during this process, but no funds have actually been taken from the account. This is a security anti-fraud feature which occurs when certain information has been input incorrectly.