Open a premium channel (e.g., Sky Sports). Watch the "ECM time" in the info bar. Anything under 0.300 seconds is good. If it jumps to 2.000 seconds or shows "no data," the free server is dead.

To enjoy high-quality content safely and legally, consider these options: Free-to-Air (FTA) Channels

If you are tired of hunting for unreliable free servers, consider these better alternatives: