What kind of cable modem do you have? For most types, there is a signal checker onboard. My motorola SB5100 is accessed this way:
http://192.168.100.1/startup.html. Yes, I'm behind a nice firewall....
But what I get is some options, one of which is Signal. On the signal page, I get Frequency, S/N, and Upstream/Downstream power levels. My downstream is 5 dBmV, upstream is 51 dBmV. Check the items with the modem directly into the wall, then recheck after adding splitters.