Whenever I try to edit a post on a SE website, it tells me it will be peer-reviewed. How can I peer-review other people's edits?
Once you get the edit questions and answers privilege (usually at 2,000 reputation), you gain access to the suggested edits review queue.
In this review queue you can review edits by other people.
Additionally, you can always review edits on your own posts (unless already approved or rejected by other reviewers).