Test a proxy before you rely on it

Check the exit IP

Request an IP echo service through the proxy and through your normal connection. The two addresses must differ.

curl -x http://<host>:<port> -U "<username>:<password>" https://api.ipify.org

Check the location

Look the address up in more than one geolocation database. They disagree, so test against the one your target relies on.

Check the target

Request the real site you will use, at the rate you plan to use. A 403 means the site refused the address, 407 means the proxy refused you, and 429 means you went too fast.

Keep the result

Note the date, the address and the status. When something fails later, you can tell whether it changed.