Web browsers make it very easy for a user to know if a webpage with a form (that includes password and credit card fields) is on a non-secure connection. The alert is meant to prevent users from entering sensitive information that could be compromised upon submit.
Google Chrome Example
If your webpage does not have this type of form on it, the browsers currently do not display any type of strong indicators to the users.
However, Google recently announced that beginning in October of 2017, Chrome will begin to show the “Not Secure” alert in two more scenarios.
Website owners are being encouraged to transition their webpages to HTTPS over the next few months. If you are planning a redesign or to make website updates, you should consider an SLL certificate so that your website visitors are no longer greeted by the “Not Secure” indicators.
Please consult with an SEO specialist before making any changes, as there are organic considerations for changing your URL structure from HTTP to HTTPS.