You don't know that. You know that you have been attacked by a friendly Civ. That is useful data - it shows that friendliness does not overrule all other reasons when deciding whether to go to war - but it doesn't say anything about the chances of this happening.
There do seem to be other stances apart from just friendly/hostile. I Jaguar rushed Germany at the beginning of a game, and Arabia talked to me soon afterwards and told me that they had heard of my military prowess, and thought that evildoers throughout the world should fear me. I suspect they wouldn't have attacked me no matter what other reasons they had.
In my current first game (Prince, Earth, Large). Civs constantly declare they are protecting city states, the thing is, I have already pissed off every city state in the world, and they are permanently at war with me. (I conquered 4 early, because other city states asked me to). Every 5 turns or so a different nation talks to me and basically says they disapprove of me preying on the weak. I have already eliminated three players, control Africa, Europe, and the Middle east. I had France ask me if I was attacking them when my Caravel showed up on the boarders of North America, and saw they only had spear men. I had Montezuma tell me he heard of my great triumphs in battle or something like that.
I only have a capitol (Kyoto), every other city is a puppet, and I support my warmongering by pillaging improvements in city state tiles. If only I could sell the useless buildings puppets build, I'd be rocking a +300 economy. My four former allied military city states are now at permanent war with me.
The point is, every civilization hates me, and I am at war with 20 city states. Literally everyone is "Hostile" on my diplomacy screen. No one has declared war on me, but they verbally insult me every chance they get to which I respond "You will pay for this in time." One leader even went so fare as to say something along the lines as "I see you cave men like to jump up and down and beat your rocks together". I wiped his civilization off the map 15 turns later.
This time around, opposing civilizations have a sense of fear. There is no "Dog Pile" effect like there was in Civ IV where everyone gangs up on the point leader. Instead they don't want to be the one to irritates me and has to face retribution. They are definitely more human or emotional then the old AI. I mean they have 20 other city states who I bet all have a "attack Japan" mission available for the AI.
At least I feel like I'm properly being a jerk to people now, in CIV4 people would have all declared on me, or whatever the major religion was would declare me a heathen and I'd be in a world war.