I fully agree with Sardathrion that there are almost always signs before actual violence, because violence is almost always an escalation. Even when there are no signs of the attack itself (e.g. a drive-by shooting), there is an history of violent escalation (insults, threats, etc.).
As for the inadequacy of martial arts with the real world, I'd first say that the many successful fights many of my martial arts practitioners friends got into are a strong body of evidence, if not conclusive proof, that martial arts "work". And some of these friends are lanky skinny geeks. In one instance, a 11 years old weighing 30kg wet defeated two seemingly brawny 13 years old (they were racketing an even smaller kid in an alley). Which goes to show that good technique goes a long way.
But having tried a few martial arts (karate, judo, ju-jutsu, iaido and aikido), I must admit that they are not all equal in this matter, and appearances may be treacherous. First, the more sport-like an art is, the less easy it will be to apply in a real fight. In this sense, Ju-jutsu, being the most versatile one, may offer you the most: whether you kick, grapple, or get on the ground, you'll be on familiar ground (no pun intended). Judo would be the opposite, being very much a sport and less a martial art. But even being good at Karate would be enough to get the upper hand in many real fights: a good efficient punch or kick is often all you need to stop the fight (contrary to what Hollywood would have us believe, most people stop fighting quite quickly; it hurts like hell, after all).
As for appearances, I sometimes wonder if this is not the source of this myth: many martials arts, when observed by a non-practitioner, seem fake. Heck, even me, when I practice Aïkido, sometimes feel like the sensei's assistant willingly went the way he was supposed to, to make the demonstration better. And my feeling is strengthened when I try the technique and it has no efficiency on my partner or me. But then I practice the same technique with a seasoned aikidoka, and I'm flying each time he applies only the most subtle pressure in the right direction. And some of these impressive techniques take a few years to fully sink in enough to be used both correctly and instinctively.
So I can understand why someone that only observed or quickly tried a martial art would think they could never apply in a real fight. But it's clearly an uninformed opinion.
And I don't wish 5 of my worst enemies to try and fight the more experienced martial artists I know… (well, actually, it would not be that bad, many of them are very level-headed in a fight and would probably try to end it with the least amount of violence).