You have to judge the value of the advice and opinions you are given. To me a critique from a stranger who I know nothing about is probably useless. It's possible an unknown could offer wisdom out of nowhere, but usually I'll need to know something about the person so I can put what they say in context. I think it's natural to give more credence to those whose works and accomplishments I respect, but it's important to listen to everyone (unless I'm just convinced they are talking out their rear end).
I think the best way to verify or assess the value of someone's advice is to get to know them. In the real world they may be someone I associate with in the local arts community. Online I can check their websites, and maybe forum posts and that sort of thing.
I also think that conversation makes a better critique. I agree that a critique shouldn't all be about whether they like the photo or not, it should be about whether the photographer is accomplishing what they were trying to achieve, and how could they do it better, more efficiently, etc... I find having this sort of conversation difficult through internet forum posts because it takes so long. Whatever end of the critique I am on there are a lot of questions and answers needed. You can present a photo with no other information, and ask for people's opinions, but that's not a critique.