We do this the old-fashioned way—email client testing. This issue is also very applicable to Outlook, which blocks images at first.
You can purchase
Litmus, which offers some nicely extensive email testing for different clients, or you can just get lots of testing accounts and see how an email sample looks in every option. It's really more of a business-needs question regarding which makes the more sense for your company. I actually don't know if Litmus offers the blocked-images-from-downloading view for Gmail or Outlook, but I personally like to test things myself in the native clients.
Best,
Edward UnthankMarketing Operations Specialist
Yesler