Why does physics attract so many more cranks than any other field? I don't know, but here's how you can identify them.
1. They claim to get huge new results without new mathematics
This never happens. Consider: Newtonian physics required the invention of calculus. Electromagnetism brought in the whole machinery of field theory, including partial differential equations, gauge invariance, Green functions, and many other things unknown to physicists of the prior century. Quantum mechanics brought in infinite dimensional Hilbert spaces and operator algebras. General relativity brought in tensor calculus and Riemann spaces. Quantum field theory brought in...itself, a mathematical smorgasbord as yet not fully characterized.
Think any of these are unecessary? Think they could be replaced by some kind of pictures or verbal explanations, if only a more incisive thinker came along? Then congratulations, you're about 25% of the way towards crankdom.
New physics requires new mathematics because, essentially, working out the results of old mathematics is a matter of effort, not creativity. Mathematics is highly structured, by definition, and if you put a few hundred smart people to work for a couple decades within a given mathematical structure, they will extract everything of physical relevance. You just can't get new wine from old grapes (and definitely not from sour grapes, see item 4).
2. They haven't mastered existing theories
Nobody advances physics without a complete mastery of the current state of the art. Most cranks think that Einstein did this, but they are completely wrong. He completed the full course of physics studies, all the way through graduate level, and then on his own he studied obsessively.
You aren't playing at Carnegie Hall without practice, and the same goes with physics.
3. They don't publish conventionally
Cranks have the idea that there have been some great physicists of the past, mainly Einstein, whose work was ignored or not published in conventional venues. This is not really true. Einstein completed his first three great papers in 1905. When were they published? 1905. Where? Annalen der Physik, a mainstream journal. Even Boltzmann, whose work on statistical mechanics met with great resistance, was a full professor and a mainstream physicist.
If mainstream journals won't publish your works of physics, they aren't works of physics.
4. They blame their failures on the attitudes of others
Cranks believe that the "establishment" is lined up against their ideas and that is why they don't succeed. When the community (largely) ignores them or fails to follow up on these "brilliant" new developments, the reason is not that the developments aren't worthwhile, but rather that the community is too narrow-minded and dominated by entrenched interests to see the truth.
Your classic crank meets all 4 of these criteria, knows very little real mathematics, and is easily ignored. However, there are some people, superficially very knowledgable, who pass 1-3 but still fail item 4, hence qualifying as 25% cranks. They publish sour-grapes books with titles like "The trouble with physics" or "Not even wrong". They think a bunch of "big egos" are standing in the way of progress, even though this never happened before in the history of physics.*
Folks, when good ideas appear you can tell. How? First, all the smartest people jump on them. Why? Because that's how they make their careers. What does any theoretical physicist have to gain by *not* pouncing on a new idea? Nothing. What does s/he have to lose? Just the opportunity for success, fame, and a place in the history books. And the second indicator that a new idea is good: it produces mountains of new and unfamiliar mathematics, see item 1. Good ideas are very fertile, and the form which fertility takes in theoretical physics is new equations. With luck, they lead to new experimental tests. Nothing guarantees that a correct physical idea has to be testable - that depends on the specific design of our universe - but of course it will be a drag if the correct theories are, in fact, not testable in practice, so that we can never know the truth.
* See http://www.theregister.co.uk/2013/02/20/carver_mead_on_the_future_of_science/