There is a simple tool named PING. Send this command to desired IP station and should be immediately echoed back. Therefore it acts as a loopback. If you can receive the echo back successfully, the path is OK.
For example, you can enter PING command in MS-DOS prompt as below in sequence.

> PC to Router ( e.g. ping 192.168.1.254 )
If there is no reply from gateway, please verify the PC, cables, HUB/Switch and gateway.

> PC to external station with IP address (e.g. ping 168.95.192.1 )
If there is no reply from external station, please verify the gateway, cables, DSL/Cable modem, and connection protocols.

> PC to external station with domain name (e.g. ping www.yahoo.com)
If there is no reply from external station, please verify the DNS setting in PC or gateway.