You can run a query on the recipients of a specific delivery and set the behaviour to 'not opened' to select those customers who have received the email but not opened it.
In your workflow, drag and drop a query activity onto your canvas and double click it to configure it
In your query select 'edit query'
Then select the 'recipients of a delivery' option
You will then need to find the email delivery that was sent out. You can search by label or create a filter and search by internal name to ensure you have the correct delivery.
At the top in the behaviour section select 'Recipients who have not opened or clicked (email) and click finish.
FYI - the way majority of marketing tools determine if an email has been opened or not is if the customer/recipient downloads the images in their mailbox once they have received the email. If the customer has their mailbox setting set to automatically download images, then even if they didn't open the email, it will show that they have opened the email. Likewise, if a customer does open the email but does NOT download the images, then there is no way to determine if they opened the email or not (unless you see that they have clicked then obviously they must have opened the email to click through).