before the modem has been authorized or in stand-by mode, if your computer is connected directly to the computer and has an ip address of 192.168.100.101 or something like that, open up the internet explorer and type in the gateway or dhcp server of 192.168.100.1, and you should get the internal diagnostic screen of the modem. In there you will get semi accurate signal readings as the modem sees it.
This is CABL.com posting #147779. Tiny Link: cabl.co/mMBH