There are two types of bad English. There's English from non-native speakers, which can be hard to read at times, but it's not a red flag. Then there's the other type of bad English. It's really hard to describe, but after you've seen it a few times, it's easy to pick up on it after a page or so. It's... a certain type of incoherence and lack of logical thinking, that superficially resembles logical thinking. The difference manifests in the structure of how they communicate, from low-level grammar all the way up to top-level organization of the paper.
Since I can't really describe what I'm talking about, I'll given the most blatant and obvious example I know of: Time Cube[1]. Even if you ignore the content, and just focus on the sentence structure, it's incoherent, often failing to parse as valid English, with a variety of ambiguous or defined referents, and freely introducing new undefined concepts. If Time Cube is 100, most papers you would see in this category are never higher than 3 or 4. But, even at that level, the lack of clarity at the structural level usually implies a similar lack of coherence at the content level.
Since I can't really describe what I'm talking about, I'll given the most blatant and obvious example I know of: Time Cube[1]. Even if you ignore the content, and just focus on the sentence structure, it's incoherent, often failing to parse as valid English, with a variety of ambiguous or defined referents, and freely introducing new undefined concepts. If Time Cube is 100, most papers you would see in this category are never higher than 3 or 4. But, even at that level, the lack of clarity at the structural level usually implies a similar lack of coherence at the content level.
[1] http://www.timecube.com/