You can use a program called DocsDiag to extract information from the modem. You'll need to temporarily change your computer's or router's IP address when running the program, but you won't have to change any wiring. If your computer is connected directly to the modem, you can use the following instructions if you're running Windows 98/98SE; if you're running another OS, you should determine how to change your PC's IP address before proceeding. (If you have a router, it doesn't matter what OS you're using.)

1. Visit the DocsDiag home page and download the docsdiag.zip file. Unzip the downloaded file to extract docsdiag.jar.

2. Change your IP address to 192.168.100.2 and subnet mask to 255.255.255.0. You can use one of the following methods if it applies to your system (note your current settings before you change them):
If your PC is connected directly to the modem and you use Windows 98/98SE, click Start, Settings, Control Panel. Double-click Network. On the Configuration tab, click TCP/IP for your Ethernet adapter, then click Properties. On the IP Address tab, select Specify an IP Address, enter an IP address of 192.168.100.2 and a subnet mask of 255.255.255.0. Click OK and OK. Reboot if prompted.
If you have a Linksys router, go to the router's setup page and select specify an IP address, enter an IP address of 192.168.100.2, a subnet mask of 255.255.255.0, and a gateway of 0.0.0.0.
If you have a Netgear router, go to menu 4, change IP Address Assignment to Static, IP Address to 192.168.100.2, IP Subnet Mask to 255.255.255.0, and Gateway IP Address to 0.0.0.0.

3. In the MS-DOS window, enter the following command:
For machines with Microsoft's Java Virtual Machine

jview /cp docsdiag.jar docsdiag

For machines with Sun's Java Runtime Environment

java -cp docsdiag.jar docsdiag

This should produce output with your signal levels and firmware version.

4. Put your IP address back to dynamically assigned by restoring the original settings you changed in step 3:
For a direct connection to the modem, go back to the TCP/IP Properties dialog box and select Obtain an IP address automatically. Reboot if prompted.
For a Linksys router, go back to the router's setup page and select Obtain an IP address automatically.
For a Netgear router, go back to menu 4 and change IP Address Assignment to Dynamic.

In addition, the following works for Toshiba modems:
To access the built in diagnostics, set your ip statically to 192.168.100.2, set your subnet mask to 255.255.255.0 and set your gateway to 192.168.100.1. Enter 192.168.100.1 into your browser you'll see the diagnostics page.

FAQ credit to Bobcat