For papers in disciplines where the authors are alphabetized, how does one decide which author of a paper to contact for more information? Suppose for the sake of argument that one doesn't know any of the authors even by reputation, so it's impossible to decide that way.
Some journals allow the authors to nominate a corresponding author and it's usually determined by a little envelope icon next to their name in the paper or on the journal's webpage. If you don't have this information, you might want to check which of the authors is still in academia (as opposed to, say, a graduate student who moved on to industry) and whether you have current contact info. Among those, just pick randomly.
I agree with what jakebeal said. That's what I usually do - I simply contact all the authors at the same time. In certain papers, it's sometimes obvious which part of the article was written by which author if you look at the other publications of the authors. In that case, you can probably decide more easily who to contact.