It’s easier to ensure accessibility compliance by monitoring the state of accessibility. You should monitor accessibility to achieve a good level of accessibility and keep it that way. If you are the only person working on a service, then monitoring might not be as necessary, or at least not as frequently. But if multiple people are working on it, it’s recommended to monitor accessibility as a whole from time to time. Mistakes, oversights and general lack of knowledge are all very much present on the websites I visit.
You can monitor the state of accessibility yourself or you can buy it as a service from someone. While technically you don't need any specific tools, having an automated tool at your disposal usually does bring its benefits. They unfortunately do cost a bit (or a lot, depending on the tool or service), so they might be out of reach for some. If that happens to be you, don't worry, you don't have to have such a tool to monitor the state of accessibility. You'll just need to do a bit more manual work than someone who can spend the money on tools.
Tools and services like, for example, Siteimprove or Deque Axe tools provide you with good automated tools to monitor the state of accessibility of the website. Depending on the package or service you get, you might get other useful quality monitoring as well. But on the accessibility side, the tool will list all issues and where those are found on the website. While it's important to remember that no automated tool can find all possible accessibility issues, having this type of tool will free manual testing time to be spent on the things automated tools cannot find. The tools might find also some language-related problems, like typos. But especially if you operate in a language that isn't as common as English, the automated tools might not be able to assist you.
Some companies also provide this kind of monitoring as a service. So if you have the money to spend, you can get the monitoring from another party and save your time for other things. Sometimes going through the automated tool and getting rid of false positives that might occur is a job that alone takes hours. It, of course, depends on the size and state of the website as well as your understanding of accessibility.
If you have a larger site or service, I recommend an accessibility audit done by an accessibility expert. An accessibility audit should also include screen reader tests of the site, which is something the automated tools cannot do.
If you are looking for an auditor, I recommend looking for people or companies that have accessibility certifications. While a certificate doesn’t prove the audit will be of quality, for example, the IAAP WAS certificate does require three to five years of proven accessibility experience.
We at Exove would love to give you an offer, so send us a message if you are looking for an accessibility audit.