A pretty reliable way to determine if someone's actions are intentional or just negligent is to yell at them as loud as you can. If they ignore you, then the action was intentional, they knew what they were doing, expected you to get upset and ignored your shout.

If someone is negligent, they will usually startle and slow down, since most people's reaction is to take their foot off the gas when something unexpected happens.

For example, a car turns left in front of you, cutting you off. If you yell and they look at you, then most likely just a bone headed move. If they just keep accelerating and don't look, then they saw you and just don't care.

I don't mind the negligent, distracted driver at all. The socccer mom on the cell phone is easy to avoid since she generally distracted, thinking about something else and her driving is usually slow and plodding and predictable. Its the jerk who sees you and is gunning for you that is dangerous. They miscalculate and your options and time frame in which to react are much more limited.