I was under the impression that u wanted some quick way to see what a passage type is before reading it.
Humanities: usually about works of art ie fiction, music, visual arts...
Social sciences: tend to focus on historical events and/or political theories.
Natural sciences: u should know what this reads like.
Note: I'm not really sure where to put philosophy because the topic is so broad. U could get a passage on literary theory (ie the effect of writing in first or third person) which would be humanities. but then again, u could get somthing on social theory (ie the benifits of communal living) which would fall right in social sciences. Yet I would consider both topics of philosophy. So that said, I think these descriptions should be used as guidelines and most dificult mcat passages are a mix of two of these categories...
Hope this helps.... And sorry for my wise *** comment earlier. I was kinda sleepy.