It requires one to read closely and evaluatively. Accept that when you review a work, it is your professional responsibility to provide an objective, carefully considered evaluation. Sort out minor issues from major issues. A manuscript should meet professional standards in terms of spelling, grammar, and structure of the piece. These are mechanical issues, not substantive.
Mechanics is the least problematic in improving a work.
Clarity of the question posed, or major thesis being developed . Often the focus will shift as people work on a topic. This creates difficulties for the author and reviewer. Identify how clear the question or thesis is. Repose the question in terms of your understanding.