You can enable Authorize.net in test mode to test configuration and if the plugin is functioning correctly.
To work in Sandbox mode it is necessary to create an account on developer.authorize.net. Production and Sandbox accounts are two different accounts, and it won’t be enough to only enable this option to switch to sandbox mode.
The Admin will also need to create a sandbox account here: https://developer.authorize.net/hello_world/sandbox/ and set Login ID and Transaction key for this account in the plugin settings.
In order to test the plugin configuration, enable Sandbox mode as explained above then go on with your test purchase and select Authorize.net payment mode during checkout.
To test the credit card method, you can use some randomly generated card numbers available on this page.
NB: Check order status during and after payment from WooCommerce panel. During payment order status is labeled as pending payment; once the order has been completed its status will change into processing or complete.