How thoroughly do referees review papers submitted to theoretical computer science conferences? Theoretical computer science conferences usually have a review period of about two months. Programming committee members have potentially dozens of papers to review, alongside their regular day jobs. How thoroughly are papers usually reviewed? What kind of heuristics are often employed to review papers faster? Does correctness of every proof get verified in detail, or do reviewers put some trust into the authors getting it right?
It varies.  As a first year PhD student I was asked to review a theory paper, even though I had almost no background in the field.  The professor explicitly said I only had to read the first half of the paper, and I only had to tell him if it looked interesting and correct.

At the same time I'm sure many reviewers do a diligent job and don't outsource their reviewing to inexperienced graduate students.

Related: http://www.phdcomics.com/comics/archive.php?comicid=1760