If I do an ifconfig I can see I have two interfaces, eth0 and eth1. However if I try and look at the network interfaces from the GUI, by clicking on the drop down menu on the top right of the screen, I can see wired connection 1 and wired connection 2. How do I know which one relates to which eth0 or 1? It's very confusing.
If you click "Connection Information" in Network Manager, you will see it.
And generally speaking, you either configure network using Network Manager or using CLI and configuration files. It is not supposed to use both.
If you change some settings for an interface in /etc/network/interfaces, Network Manager will stop managing that interface.