I think what YouTube does would be sufficient. There’s a noticeably different video progress bar colour (yellow instead of red) and a large "Skip Ad in __" in the corner, plus the advertiser information on the side.

Reddit could do this by putting a “Paid advertisement” watermark in the corner or putting “Advert” where the upvote/downvote buttons are and colouring it some noticeable colour, like yellow, and I would be satisfied with that.