No email tracking can work all the time but this will work in as much cases as possible, is completely free, and is so seamless you don't even know it's there until you need it. Enable it and I guarantee you will get some interesting stats about the people you email. It's definitely helped with my follow ups and replies.
You kind of dodged the person's question. What do you use to check that emails are opened?
Sorry. The extension uses a hidden tracking pixel to check if emails get opened.