A word of caution on accessibility validators/checkers. They catch some mistakes, but they cannot replace human review. For instance, they can check if you are using headers in the wrong order, but cannot check if you missed tagging a heading as such. If you find yourself bolding 1-3 words at the top of a paragraph or list, it is probably a heading and needs the appropriate tag. Definitely use validators/checkers, but in addition to reviewing the content for accessibility.
These validators or checkers help catch accessibility errors on your web pages.
Unfortunately, there are no free document checkers. However, there are tools within Word, PowerPoint, and Acrobat for checking your documents for accessibility.
More step-by-step instructions at WebAIM: Word for Windows 2016 Using the Accessibility Checker
Errors appear in a panel on the right. Open the error with the arrow on the left of the error. Clicking the object of the error will take you to the slide and highlight the issue. Information on correcting the issue will also appear in the panel.