One marker to watch out for is a kind of selection effect.

In some fields, only 'true believers' have any motivation to spend their entire careers studying the subject in the first place, and so the 'mainstream' in that field is absolutely nutty.

Case examples include philosophy of religion, New Testament studies, Historical Jesus studies, and Quranic studies. These fields differ from, say, cryptozoology in that the biggest names in the field, and the biggest papers, are published by very smart people in leading journals and look all very normal and impressive but those entire fields are so incredibly screwed by the selection effect that it's only "radicals" who say things like, "Um, you realize that the 'gospel of Mark' is written in the genre of fiction, right?"