2. Ask someone else to summarize the book/film you're interested in.

a) If said person is an absolute fan of the book/film, they will fangirl/fanboy over it until you're fed up with merely hearing its name. Stay away from these people, as they can often insist so much that they'll make you avoid an otherwise interesting read/view. It's not their fault, and I can honestly say I've done it myself on occasion – they just like it too much!

b) If said person is an extreme hater of the book/film, you won't get much aside from "Urgh, it's utter crap!" out of them. You could ask them why they think so, but you'll risk spoiling yourself accidentally because haters usually disregard the need for "spoiler tags".
Personally, I haven't found a book or film that I truly hate, so I don't believe anyone who says they hate something. Everything has something good, which is why "dislike" is more appropriate than "hate" or –God forbid!- "loathe".

A very common practice is hype. In some cases, it surpasses and blows up the true worth of the book/film. Therefore, try to keep an open mind when reading/viewing and leave most of the subjective opinions you've heard before, behind. Judge using your own brain, not others'. Unfortunately, some books/films divide people so clearly that it's hard to judge beforehand if it's really worth it. An example of this would be the Twilight series (both books and films): it's difficult to find someone on the fence about it.

Also helpful is reading a proper, spoiler-free review (off the internet, the back of the book etc.) and judge for yourself. Or get someone level-headed to give you a good summary, without sounding too subjective (and influencing you). Also, be sure to warn them you don't want any spoilers – after all, why ruin everything by knowing beforehand what's going to happen next?
Also, be sure to ask if the book/film addresses any topics you may find disturbing, such as rape, violence, necrophilia. Any normal person will calmly answer your question without freaking out.