You know it's a bad review if , after reading it, you don't know what the strong and weak parts of the product are. These are absolutely basic questions which many reviews simply fail to answer. In the best cases you should feel as if you've had a personal demonstration of the product. It can be difficult and time-consuming to actually learn a program, so many reviewers don't. You know it's a bad review if it only tells how fast a program completed a task.
Speed is relative.
A program may be incredibly fast at printing, but incredibly slow at getting the pages to the point where you print them. It doesn't matter if a program is lightning fast at something esoteric if it takes 6 keystrokes to perform a common task. Yet this kind of reviewing requires a knowledge of more than one program and an understanding of what users actually do most often. You know it's a bad review if it uses the same canned illustrations you've seen for the product in every other review in every other magazine. If an author hasn't taken enough time to create real documents or enter real data, they may not have the faintest idea what the program is like.