ChilliDB can track if your Messages are being opened or not through the "Track Open" checkbox on the "Tracking Options" section of the Send Message window. These checkboxes are shown by clicking the 'Show Tracking options' link from the actions menu.
When each email is sent out, we automatically add a piece of code that requests a tiny, invisible image from our web servers. So when a reader opens the email, the image is downloaded, and we can record that download as an open for that specific email.
The opened status of the email can be checked by viewing the note created after the email was sent.
It is important to understand that the open rate is not a 100% accurate measure. Recording an 'open' can only happen if the reader's email client is capable of displaying HTML with images, and that option is turned on. So if you are sending text-only emails, there is no way to record open rates. Similarly, people reading your HTML email without images showing will not be recorded as opens.
Another issue is that your readers may have a preview pane in their email client. That preview pane might be displaying your email automatically (and therefore downloading the images) without the reader ever having to click on it or read it.
So you should never take your open rate as a hard and fast number, because you can never know the true figure. It is much better used as general guide, and as a way of measuring the trends on your email campaigns.