A check answers three questions: does the proxy accept the login, what address does the request leave from, and where is that address. The simplest checker is one command that fetches a page which prints the visitor's IP.
A check tells you the proxy works in general. It does not tell you whether your target accepts it; only a request to the target does. Every PiratProxies plan has a test button that runs the check from our side, which separates a problem with the plan from a problem with your setup.
Read next: How to test a proxy