You've got to use reviews and video content. Get really acquainted with a few reviewers and what games they really like, what they don't, and their general mindset. Even if a reviewer doesn't like a game, if you understand their taste and preferences you can even tell when you might like it. Cross reference with general public opinion, or perhaps the development history of the studio and if you've played and enjoyed their previous games.
But basing anything off ONLY screenshots and trailers is a horrible trap and piracy isn't the exclusive way to find that out.