How do i see if an email which was sent through decilehub was opened by the recipient?

That is surprising since it's a standard feature on competing products like Pipedrive, hubspot, etc.

Hub is optimized for deliverability @Michael Coates

To do email tracking by default, the system would need to send a tracking pixel which often pushes the email to spam. That being said, you can enable it by tracking clicks on links, which is by far the most common and important use-case for this functionality, especially in a fundraising flow. Simply setup an action for all the links in your email template and the prospect will automatically go into a different stage of your choosing. 

I'll also add in @Michael Coates

that unless an organization can dedicate huge resources to combatting "bot" clicks, email open tracking will unfortunately never be accurate. Email open tracking obfuscation has been going on for a while by most major ESPs. If you are interested in learning more, this article is a nice dive into some of the challenges behind it: https://developermedia.com/email-open-rates-misleading-metrics-best-practices-2/ I'll emphasize a quote from that: "Nobody can accurately report on open rates (don’t believe them if they tell you they can)"