Instead of hunting for a checker, get organized:
The safest “Microsoft Product Key Checker” is the activation wizard built into Windows itself. Everything else is either useless, dangerous, or both. microsoft product key checker
If you need to verify a key without installing it, certain reputable community tools can analyze the key's "Product ID" (PID) to determine its origin: Find your Windows product key - Microsoft Support Instead of hunting for a checker, get organized: