People are altogether too cavalier about ascribing motivations to others. If sometakes takes an action A, and someone wishes to accuse them of motivation M, the person will ask themselves, "Does M explain why A was done?" If so, then the accusations fly!
Yet that question is seriously fallacious and will only produce a true statement accidentally at best. The correct question is, "Given that a person has motivation M, would they consider A to be their best action?" (One must also consider other circumstances, such as intelligence, but this is the basic question.)
Consider an example. If a very rich multi-billionaire publically gives a thousand dollars to a children's charity, one may explain that with "Obviously he really cares about the children and wants to help them." But this is not really the logical conclusion to make from this action. One must ask, "If this billionaire wants to help the children, will he consider giving a thousand dollars to be his best action?" In the artificial absense of other information caused by the way any example like this is simplified, the answer to this is no. If the billionaire wished to help the children, he would give substantially more money, not what is to him a mere pittance.
One is much more likely to be correct to conclude this is a gesture designed to improve his standing in the public eye, as most people percieve a thousand dollars to be a significant donation, then something truly intended to be helpful.
To take a real world example, consider the argument that the war in Iraq was "Blood for Oil". This argument might seem to explain the aggression... but it horribly fails the correct motivation test. Clearly, if the war was all about oil, and had nothing to do with Iraqis, I think it's a good guess (based on history) that we would establish another tyranny (because "what else do the Arabs understand, right?"), one friendly to the US, we'd let the new tyranny sort out Iraq with our military support, and we'd be concentrating monomaniacally on getting the oil wells operational, to the exclusion of everything else. As a condition of the support of the new tyranny, we'd have a significant, direct interest in the new oil wells. This is not a reasonable interpretation of the news coming from Iraq, even if you pay attention only to the "majors". Even if you try to add "covering up" to the mix of motivations, it just doesn't play out; if this was our end goal, there are much, much simpler and more reliable ways to attain it. With a bit of work, we could probably have even done it in a seemingly-legitimate way... after all, France and Russia seem to have managed that trick. (For bonus points, apply this test to their involvement...)
I see no reason at the moment to believe anything but that Bush's primary motivation in Iraq is to genuinely help the Iraqis and to create a stable government there, as one step in a long-term attempt to reform the entire region. Here I use "primary" to mean the one that is driving the majority of the actions taken by the US, not necessarily the one that Bush would consider most important in his private thoughts. We can argue endlessly and fruitfully about whether this was a good idea at all, whether this is the best way to help the Iraqis, whether the motivation is being correctly translated into action, whether there are devious sub-motivations in play from Bush or others, etc. But I think the primary motivation is almost beyond argument; no other motivation that I can think of produces the actions we have seen so far.
(Update Dec. 2006: This last paragraph has bothered some people, so let me say it another way: If you take the motives imputed to Bush by other people, then you don't get the actions taken by Bush. The putative revenge motive would have long since been fulfilled, yet we remain. The oil logic given above holds true; we are yet to grab the oil and run. The only one that comes even close to explaining the situation is an attempt to possess valuable strategic bases in the Middle East, and even that really doesn't account for why we're still there. I really do not see a better motive than the ones he claims in public.)
Assuming a reasonably rational actor, it is usually pretty easy to divine motives based on their actions, unless they are in one of those rare situations where high levels of deviousness are truly desirable (usually in constrained game situations, like in Chess, or in violent conflict like a battle). In most of life, you can usually get to the motivations underneath, if you ask the right question.