"Peer Review"
FYI:Peer means a person who is of equal standing(or status 
or designation) with another in a group 

So Peer review means you are just reviewing your co-testers 
or team mates work and that means just checking whether 
they have done their given jobs correctly or not. 

For this first you need to have the knowledge or 
information of what you are going to review.Then 
automatically you will know what should be checked.

For example:You need to peer review the Testcases written 
by your team mate.

First thing you need to know is -- what scenarios were 
given to him  to write testcase.
Then you should know how the testcases should be written 
and based on your skills you can review them and give 
comments or clarifications .
In order to get the above information about testcases you 
need to refer to the functional specifications or the 
coresponding documents related to that testscenario.