There is an option in the plugin to test out all your settings in a live test. To make sure all is set up well, before you go live and start working with accounts of real clients.
Follow the steps given underneath the API Key fields, they will redirect you to the correct page in your Stripe Account.
Make sure “Viewing test data” is enabled in your Stripe Account. You can doublecheck if it is enabled, if you see “Test mode client ID”.