Earlier this year, I discussed the different kinds of editing and feedback. So, today I'll talk about what to do if you're the person who gets asked for feedback on someone's writing. You may have formed a critique partnership with a fellow writer, where you give feedback on each other's work, or someone who knows you know something about writing might ask for your advice. You might also be able to apply some of this to evaluating your own work.
The first thing you need to do when asked for feedback or a critique is determine what the other person wants from you. They might really just want a pat on the head or encouragement to go forward, and if you give a detailed critique, it can get ugly. I often ask people for what I call a "sanity check" -- yea or nay, does this work at all, is it worth developing further, should I stop trying to write while taking allergy medication? In that case, I don't want detailed constructive criticism, just did you like it or not, was there anything major that worked or didn't work. I might sometimes send sections of a book out with a specific question in mind -- which of these two versions works best? Does the other person want a detailed critique, a line edit or proofreading? It's okay to do some negotiating here because it's your time. It's rather pointless to do a detailed line edit or proofread on something that's still being revised at the plot level, so it's unreasonable for someone to expect that level of editing on a first draft. If you establish expectations -- on both sides -- up front, you're less likely to end up with hurt feelings or misunderstandings.
If it's a general beta read or critique where you're not doing final proofreading or answering a specific question, what do you look for? A good guideline is to look for the "hey, wait a second!" things -- anything in the story that gives you pause. Would he really do that? Why would he do that? Could he actually do that? How did he do that? How does this work? Why is this happening? Why didn't this happen? Can't they just talk about it? This includes things you don't believe, where the world building or characterization isn't strong enough to make you suspend disbelief. It also includes things you flat-out don't like -- you had me until this. Or it can be things you didn't understand. Look out for boring parts where your attention strayed or scenes that seem repetitive (they're doing that again?). Even if you're not doing a line edit, you can point out obvious problems like a word that pops up way too many times (how often does she grimace?) or a word that's being consistently misused (I once did a critique where I had to ask if the story was about soldiers or actors because the word "troupe" was being used throughout, but the context seemed more military, and I wasn't sure what the writer was going for -- the militant wing of the USO?).
Don't forget to point out the positives in addition to the negatives, and not just to ease any fragile egos. Knowing what works is important because the writer needs to know what to keep when rewriting or what to build on. A character who became one of the most popular in my series initially had a smaller role, but when I was sending chapters of the first book to a friend for feedback as I wrote, she really loved the character and wanted to know more about him. That told me there might be something there, so I gave him more attention and put more thought into his story.
Criticism should be constructive in the sense that it gives the writer something to work with. You don't have to offer a solution to the problem because that's up to the writer, but you need to give more detail than "this sucks" or "I hate this." At least give a "because." "This scene didn't work for me because your character, who seems like an intelligent person, was suddenly making idiotic decisions. Is there a reason why he lost his ability to think rationally, or do you just need him to make these bad decisions for your plot to work? You either need to justify his decisions or think of another way to make the plot work."
Finally, you need to check your own ego. It's the author's story, not yours, and even if you put in hours of blood and sweat on the critique, it's up to the author how much or how little of your feedback to use. I've had people resist making any changes, and their books went nowhere, and I've had people have great success in spite of disregarding my critique (I still think it was a plot hole, but it was a genre trope, so I guess it was a plot hole acceptable to the publisher). It's not your book, and once you've said your piece, it's out of your hands. You do, however, have the right to decline the opportunity to do future critiques if the writer is unpleasant to you about it or disregards all feedback with the sense that they really just wanted to be told how brilliant they were.