For web content there are many web-based accessibility checkers, as well as browser-based plugins, that will check for accessibility errors in HTML content. Though these tools are a good first step for identifying potential barriers in web content, there is much variation in coverage and accuracy across the many tools available. It is generally a good idea to test with at least a couple. Even when multiple checkers are used, there are a variety of potential barriers that automated checkers cannot identify with any certainty. In general, any issues that involve meaning will require a human to make a decision. For example, all accessibility checkers can determine whether an image has alt text or not, but none of them can tell if alt text accurately describes its associated image.
Here are a few accessibility checkers you can try:
Many current document authoring tools have accessibility checkers built into them, though sometimes they get buried in a sub menu. Be sure to look for an accessibility checker in whatever authoring tool you are using and run it to identify any accessibility problems that might be in your document.
Microsoft Word and Adobe Acrobat Pro have fairly good accessibility checkers built in that should be run, with necessary adjustments made to the content, before making a document publicly available. Other Microsoft and Adobe products have accessibility checkers as well. The ones mentioned here are the most commonly used.