How to determine how much current and or voltage a given circuit "needs"?
As you may have gathered from the other responses, this is generally not a trivial exercise.

At the individual component level, each manufacturer publishes a datasheet that lists, among many other things, the max and min supply voltages and the expected current for at least one recommended supply voltage.  Add up all the components on the same internal power bus (don't forget passives) and back-calculate the current going into each regulator at the voltage that feeds the regulator.  (more datasheets)  Add up all of those, back-calculating again through cascaded regulators, until you end up at the power input.

As for the input voltage selection, they may have chosen something just higher than the highest regulated voltage + dropout of that regulator.  I suspect though, that a laptop adapter would be just enough to charge the battery and the rest of the computer then runs off the battery terminals, even if there's not actually a battery plugged in.

Most of the time, this process is much more iterative in design than in testing.  There may be a total power budget to start with (probably specified in watts), then parts are chosen somewhat by experience to try and add up to less than the budget.  If it's over budget, then some of the parts are substituted for more efficient or less capable ones and the total power is calculated again.  Once the numbers work out, in many more ways than just power consumption, then it goes to the first prototype.

In some designs, the power budget is a major factor in the design, like an all-day netbook.  In others, it's more of an afterthought so long as it can still be cooled adequately, like a high-quality architectural drafting engine that is used both in the office and on a job site.

Of course, this is grossly simplified, but you get the idea.