This is step 6 of our onboarding guide to becoming a reviewer
Plugins
Plugins for automated checking against the WordPress Theme Review Coding Standards and requirements are essential for the review.
The plugins can not replace a manual review, but they can help us do better and faster reviews.
Plugins catches things that we did not see, things we forgot to check, and gives us hints for things that may or may not be a problem.
If these plugins shows that there are errors in the theme, the errors need to be confirmed manually.
Example error report:

Our second plugin, the Theme Sniffer, is recommended for users who are familiar with PHPCS and the WordPress coding standards. The plugin can be downloaded from GitHub.
Example content
We also need some content on our test install to be able to test how the theme handles posts, pages, widgets and comments.
The Theme Unit Test is an .xml file meant to be imported via the built in WordPress import tool.
Note:
The instructions on the codex page are meant for theme developers who wants to test their themes.
As a reviewer, you do not need to follow each step listed on the codex page.
Download the file and import the content to your test install. Remember to import the images.
The Theme Unit Test Codex page: https://codex.wordpress.org/Theme_Unit_Test
The Theme Unit test on Github: https://github.com/WPTT/theme-unit-test