Art critics write decide how they feel about the subject. They analyze the work and look for themes and messagesthat the artsit was trying to convey. They follow a basic set of art stardards by which they judge artwork. These include unity, movement, and emphasis.
Critics can either receive a bachelors degree in a writing-related degree such as communications, journalism, or English. They also need a degree in art. Unless they are self publishing in which case they require no formal education because their opinions are a reflection of themselves, not their employer.
Personally, I don't feel like I could ever critic another persons artwork because I feel that everyone should be able to express themselves and there's no wrong way to do it. I wouldn't know what to look for, and or how to compare it to other art work either. It is definately not a career that I am interested in.