Windows: `netstat -ano | findstr :PORT`
Windows: `Test-NetConnection localhost -Port PORT`
Windows: `Get-NetTCPConnection -LocalPort PORT`
Linux/macOS: `ss -tuln | grep :PORT`
Linux/macOS: `netstat -tuln | grep :PORT`
Linux/macOS: `lsof -i :PORT`
Linux/macOS: `nc -zv HOST PORT`
Linux/macOS: `telnet HOST PORT`
Check firewall rules for the port
Check service status for the application using the port
Use `nmap -p PORT HOST` to scan the port
Use `curl http://HOST:PORT` for HTTP ports