In fields that alphabetize authors, which author to contact

Purely alphabetical author-list conventions and no designated contact author tends to show up in theoretical fields that usually have fairly short author lists.

As such, if you're going to contact the authors and don't know which to contact, why not put them all in the "To" line of your email?

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.