Sometimes, when you have enough information about a situation and a person, and if you are reasonably empathetic, you can usually get a pretty good idea about other people's motivations and the needs and concerns that inform their positions. That's how it works for me, at least. If, on the other hand, you are empathically challenged or have insufficient data on which to decide or just can't be bothered to find stuff out, there are some shortcuts.
1. Use defaults such as the one I employ: "Never attribute to evil anything that can be adequately explained by stupidity." This does not apply to GOP pundits or politicians, of course, or to anyone else who has repeatedly overriden the default. Or you might assume ignorance rather than deceitfulness until it becomes apparent that facts don't matter. These defaults work because people are, in fact, more likley to be stupid or ignorant or both than to be outright evil or deceitful. Also, if you are wrong, the error probably won't hurt you as mush as if you assumed evil and deceitfulness when in fact you were dealing with stupidity and ignorance. It's hard to back down from attributions of evil and deceitfulness. "I'm sorry I thought you were evil and deceitful when all along you were just stupid or ignorant."
2. If you are a social retard, do what all social retards do and project your own motives onto others. This won't help you figure anyone else out, but it will let the rest of us know where you're coming from.
3. Pretend that the other person is completely rational and assume that he or she desires all the consequences of their positions or ideas or actions. If Donald Rumsfeld went into Iraq with inadequate forces, it must have been because he intended for there to be security problems after the invasion. That way, the war could be extended indefinitely so as to bolster the administration's use of a war footing to justify all kinds of shit. There was never any intention to end the war and to leave Iraq secure. See what an interesting result you get? That's way more interesting than figuring that Rumsfeld was a big dummy. We all know he wasn't, but that's how he had to play it when folks started to figure out what was going on. What could he say? "I'm not incompetent; I'm evil!"
4. Pretend that the other person is a zombie without self awareness. This works for the religious right, birther types, and Rush Limbaugh/Sean Hannity/other wingnut viewers/listeners. There's really no point in trying to fathom their motives. Why does a sheep go ths way or that way? Ask the shepherd. Just go to their leaders' motives.
5. In the case of journalists (real ones, not FOX, in which case see #4), assume that you are dealing with someone really lazy with a cheap boss. Why is he reporting on this mindless drivel? Why is he regurgitating press releases? He's lazy, and he isn't supposed to spend time and money digging into stories.
These can be real time savers.