It depends on the university but generally it is split in two: academic and clerical, though which order they see your application will certainly depend on the university itself. Clerical (admissions) selectors are general, tend to be grouped together in departments and faculties (school of humanities, for example) and will focus on the non-academic aspects of your application; checking transcripts and what like. Academic selectors will then check academic aspects of your application such as essays and research proposals to ensure your suitability. You should be able tell in some cases which academic selectors are making decisions, depending on the size and scope of your prospective department/faculty. For example, if you submitted a research proposal, suggesting that staff member x was the most suited to supervise you and no other member of staff could, then it is pretty safe to assume that - if you were given an offer - staff member x and the programme leader, who probably tries to ensure all staff are allocated someone, gave you the go-ahead.
If you apply for a really broad programme in a large department then it will be almost impossible to tell unless you are told once you get there, and I assume that there is a meeting organised with by programme leader with a number of staff present, who will go through the applications, choosing the best students; similar to meetings staff have at the end of the academic year to decide your grade(s).