For an online proxy checker, free tools like the checker on ProxyScrape let you quickly test whether a proxy is alive, check response time, and see the anonymity level, whether it is transparent, anonymous, or elite. If checking a larger list, look for a tool that supports batch checking to save time over testing one by one.
For more detail, good checkers also show geolocation, protocol type such as HTTP, HTTPS, or SOCKS, and whether the proxy leaks your real IP through headers.
here is where testing on more than one tool matters, since results can vary, and if you are technical, you can also curl through the proxy and check the response headers yourself.