Check for a read receipt, if your email service supports it and the recipient allows it
Use email tracking tools or extensions that show opens and clicks
Look for a reply or follow-up from the recipient
Check whether the recipient clicked a tracked link in the email
See if the recipient downloaded an attached tracked file, if your tool supports that
Review delivery and open reports in your email marketing or business email platform
Note that many email clients block tracking pixels, so open tracking is not always reliable
Note that read receipts are often optional and can be declined
Assume you cannot know for sure unless the recipient confirms it