If you are re-reviewing a manuscript (after round 1) and the manuscript of the current round has no tracked changes, you can use Microsoft Word to compare the manuscript of the current round to that of the previous round (however, the author also should have provided a point-by-point response, outlining what he has changed, see As a reviewer, where do I find other reviewer comments and the authors' responses to the previous reviewer comments?).
Before you do so, check the supplementary files to see if the author has uploaded a tracked changes version as suppl file, as we recommend.
If not, proceed as follows.
Getting the revision from the previous round
In the Reviewer menu, click on the Archive tab (at the top) and find your previously completed review for the article in question. Go into the review and download the manuscript. This is the revision from the previous round.
Getting the revision from the current round
In the Reviewer menu and on the Active tab, find your incomplete review for the article in question. Go into the review and download the manuscript. This is the revision from the current round.
Microsoft Word > Tools > Track Changes > Compare Documents
Once you have the 2 manuscripts to compare, go Tools > Track Changes > Compare Documents, select the 2 doc files, and the result will be the same as "tracked changes".