One way to tell whether an email has been opened or not is to include a 1x1 transparent pixel that is hosted on a server you control. If the email client the recipient is using automatically loads images, you will get a hit to your webserver for the transparent pixel. This only works if display of images is enabled, so it's not 100% reliable. Some email clients turn off automatic loading of any external content by default for privacy, but it still has value and will help you track opens.

Also, set up seed accounts on different email providers and subscribe them to all of your lists, this helps you get an idea for what other people are seeing when they get the emails. If it's going to a seed account at gmail, and getting sent to the spam folder, you can try tweaking the email and work around the issue with some A/B testing.