Log in to the MailChimp account and click on “Audience” in the top navigation.

MailChimp audience

 

If there are multiple audiences in the account, select the relevant audience.┬áThen for the relevant audience, open the “Manage Audience” options, and click “Settings”.

MailChimp audience manage settings

 

From the list of settings, click on “Audience name and defaults”.

MailChimp audience name and defaults settings

 

Then under “Form settings” you will be able to see whether or not double opt-in is enabled.

MailChimp audience double opt-in checkbox