Exclusive reliance on logical consistency
Any work that proudly proclaims its logical consistency, to the exclusion of other forms of evaluation, is probably bullshit. Logical consistency is not by itself all that impressive: UFO-ology, homeopathy, flat-earthism, religion, conspiracy theories and paranoid delusions are all logically consistent. Logical consistency is necessary but not sufficient to exclude bullshit.
Any purely deductive argument can be denied by denying one of its premises. (Keep in mind that "almost everything" is as much of a denial of "everything" as is "nothing".) By definition, the premises of an argument are not deductively justified.
For example, a core premise of ordinary arithmetic is that zero is not the successor of any natural number. However, if you deny this premise, you just get modulo arithmetic (like clocks: 23 + 1 = 0; zero is the successor of 23). Similarly, if you deny Euclid's parallel postulate, you get not falsity but either bounded or unbounded curved geometry.
Note that philosophy, just like theology, depends almost exclusively on establishing logical consistency, which is why we shouldn't be surprised when quite a lot of philosophy turns out to be bullshit.
Rationality combines logical consistency with sensibility. Sensibility is agreement with the evidence of our senses. Any notion that is insulated from and irrelevant to our senses is just as nonsensical as a notion that contradicts our senses.
Conflating uncertainty with epistemic nihilism
The argument goes, "We don't know anything with certainty, therefore don't know anything." If you want to define knowledge that way, fine. But if we don't know anything, then everything is bullshit, including what the speaker is proffering: If we can't make any distinctions at all, then how are we supposed to distinguish his tasty bullshit from the alternative nasty bullshit? Indeed, if a speaker asserts such nihilism, how are we to distinguish his words from "goo goo gaa gaa"?
Any time someone argues for epistemic nihilism, you can confidently bet that they will follow it up with this kind of argument: "Since we don't know anything, your assertion that you know that no God exists is false. Therefore, we know that God exists." Stated so baldly, the fallacy is obvious.
We don't really know, but...
The proper place for the period in a sentence like this is immediately before the "but". I'm not interested in your fantasies, delusions, wishful thinking, stupidity, ignorance and errors. Tell me what you know or peddle your bullshit elsewhere.
Complicated grammar and gratuitous jargon
When you read something, and it looks complicated and hard to figure out what the author is actually saying, you should immediately suspect bullshit. The author is at least writing poorly, and if his writing is unclear, his thinking is at least unclear. Good expository writers use simple declarative sentences and relatively uncomplicated grammar; they leave the linguistic flash and bang (cough James Joyce) to the writers of fiction.
Often it turns out to be the case that the writer is hiding a trivial or obvious idea behind a complicated expression. In Surely You're Joking, Mr. Feynman Feynman tries to read a sociology paper at a conference.
I started to read the damn thing, and my eyes were coming out: I couldn't make head nor tail of it! I figured it was because I hadn't read any of the books on [the provided] list. I had this uneasy feeling of "I'm not adequate," until finally I said to myself, "I'm gonna stop, and read one sentence slowly, so I can figure out what the hell it means."You should also be wary of undefined jargon, especially outside a purely professional context (even often within a professional context, especially philosophy). If an author is using a term in a manner apparently at odds with or unrelated to its dictionary definition, you should be immediately suspicious. Even the most professional jargon retains some connection with ordinary meaning.
So I stopped — at random — and read the next sentence very carefully. I can't remember it precisely, but it was very close to this: "The individual member of the social community often receives his information via visual, symbolic channels." I went back and forth over it, and translated. You know what it means? "People read."
Dumbing it down
Expressing things clearly does not entail actually dumbing things down. Clarity enhances precision; To deny precision and accuracy for the sake of clarity is just an admission that the speaker doesn't understand what he's talking about.
If you can read well and remember learning algebra well enough to know at least that it's not magic when someone says that the square root of x2 + 2x + 1 is x + 1*, then you should be able to learn the at least the basics of anything, including quantum mechanics.
*And if you do think it's magic, you really should go back and study. Basic algebra isn't that difficult.
Argument by analogy and metaphor
Analogy, simile and metaphor are great literary devices, and they're somewhat useful as explanations, providing a starting point for understanding a topic, but analogy is unsuited for argument or proof.
Anyone who starts, "God is like..." or, "Intelligent design is like..." can be counted on never to show that the analogy is accurate or true.
My opponents are idiots
Any time a writer treats his opponents as complete idiots and imbeciles, you should suspect bullshit*. There are exceptions, but it's rare for a class of people to be entirely stupid about their life's work. If someone's argument depends on their opponents making very obvious, stupid mistakes, you have to wonder if they are representing their opponents arguments accurately.
*The irony has not escaped me.
None of these elements are absolute proof of bullshit. Depending on the subject matter, the context and the intention of the speaker, any of these elements might be necessary to describe some point of view correctly and optimally. It's sometimes valuable to focus on logical consistency. Pure speculation, identified as such, has its place. Some ideas really are complicated and require specialized, counterintuitive jargon. Analogy is sometimes a useful tool for understanding a complicated idea. And yes, sometimes the proponents of some idea really are complete idiots.
I offer these points not to prove what is and isn't bullshit, but rather to arouse skepticism and provoke careful investigation.
Surely You're Joking, Mr. Feynman
Cargo Cult Science
Logic and Fallacies