Before paying for proxies, test them. Three quick tests take five minutes and save you from buying junk. First — liveness: a proxy must respond, not hang. Second — speed: measure response time, it should be under a second. Third — anonymity: sites must see a foreign IP and country, not yours.
Testing proxy liveness is simple: request any site through it and check for a reply. Timeout means a dead proxy. An online proxy test shows speed and country too. For SOCKS5 the test is the same, just a different protocol.
Remember: free lists have few alive proxies — half are dead within an hour. A paid pool with liveness checks and refresh every few minutes is what you need. Then proxy testing becomes unnecessary: the service already checked for you.