I'm Zuzanna, from the team here at Shopify :)
When it comes to turning on test mode for your Shopify Payments gateway, there are only certain credit card combinations that will actually let an order go through, you can see more details about this on this page of our Shopify Manual. This means that when people try to checkout with their own credentials while your gateway is in test mode, they will be declined. You can see any failed attempts in your abandoned carts section.
If you are testing things in test mode and use the correct credentials then an order will appear in your admin with a notification that lets you know it's a 'Test Order'.
Finally, when it comes to testing orders, you won't incur any credit card fees!
I hope this helps! If you have any more questions or concerns feel free to reach out to us here :)
Have a great one,