It looks at the request headers.

When you navigate to https://i.redd.it/dl34o62azctc1.jpeg, Chrome sends (I'm doing this on Chrome because I think I've made some hacks in Firefox to fix this)

  GET https://i.redd.it/dl34o62azctc1.jpeg
  Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9

and Reddit responds with a redirect to https://www.reddit.com/media?url=https://i.redd.it/dl34o62az....

But when the browser loads the same image from an <img src="..."> tag, Chrome sends

  GET https://i.redd.it/dl34o62azctc1.jpeg
  Accept: image/avif,image/webp,image/apng,image/svg+xml,image/*,*/*;q=0.8

and Reddit responds with the actual image.