Here's what you do. Invite them to lunch and very slowly stab them in the eye with a fork. I mean, wow, that's got to hurt, and holy cow, what kind of a dick would do that? But it's OK. Once the retina is reattached and the searing corneal pain ends, you and your new bud will have moved into a new area of trust and it will be worth it. Actually, that's a terrible idea, but I do know it honestly does say a lot about the content of someone's character if they can still not treat you too terribly when they're under duress, so I stand by my advice.
They Say "All Is Fair in Love and War" as Rationalization for Bad Behavior
I've written a lot over the years about stupid expressions. This one expression, however, never really pissed me off until I thought about how (in its use today) it's nearly impossible to say or believe it without being morally bankrupt. If you know someone who enjoys regaling you with this phrase, take two giant steps back. Then grab the rubber mallet you keep 4 feet behind you at all times, take two giant steps forward, and beat them to death with it.
Conventional hammers work too.
Let's think about what this phrase means. We'll start with war, which, as most people agree, is pretty awful. Why? Because it is a source of death and destruction, and, as the phrase indicates, despite Geneva Conventions and whatnot, there are no rules in war. The only rule is to win despite all the death and destruction it requires. OK.
Next, this expression equates love with war. Now, that is not necessarily problematic. After all, the point of the phrase is to say that this emotion we call "love" can be as destructive as war. It can drive people to win at all costs. It's not a rationalization. What I'm talking about are the people who use this phrase today when they're unmoved by the inequities of the world. People who see bad behavior and just raise one eyebrow and offer, "Well, yeah, all's fair in love and war," as if that's fine.
"Did you hear the boss gave the promotion to his mistress?"
"Well, all's fair in love and war."
"That rich donor was moved to the front of the hospital's organ transplant list."
"Yeah, well all's fair in love and war."
If you encounter one of these people, try telling them something less generally horrible and more specific to them. You'll get a different reaction:
"Do you know that someone took your parking space?"
"Well, all's fair in ... wait, really? WHAT THE FUCK?!"
They Make Telling Assumptions About You
Here's another way to get to know someone quickly: Watch how they treat you. Where are they defensive? I've found that, typically, people assume that their own worst qualities are present in others. Liars tend to think you're lying. Passive-aggressive people tend to think you have a hidden agenda. Cracked's own Felix Clay tends to think you have a puppy under your bed that you abuse sexually.