Check the image’s metadata

Some image creators embed crucial information about their images into the file's metadata (also known as EXIF data). 

It's easy to access this information from your Mac or PC desktop

Viewing EXIF data in Windows is easy. Just right-click on the photo in question and select "Properties".

View EXIF Data Using Preview in macOS
On macOS, by opening up a photo in Preview. Once open, click on "Tools" up in the menu bar at the top.

From there, select "Show Inspector".

Click on the "Exif" tab if it's not already selected.

You'll then see a slew of advanced information about the photo, including the various camera settings that were used. It'll even tell you if the flash was used or not.