Point your browser at HTTP://192.168.100.1 if you have a Motorola Cable Modem. That'll bring up the internal status page. Check the Signal Level page and write those figures down. You want the downstream SNR to be above 30dB (or close), and the downstream power level to ideally be between -8 and +8dB (although -15 to +15 will work but not ideal).

The most important figure is the upstream power. This ideally should be below 50dB but up to 53dB is acceptable. 55dB is generally the limit... and indicates problems with the cable network impeding the return path. Upstream SNR (Signal/Noise Ratio) should be 29dB or above. Also, run a speedtest at someplace like dslreports.com and get an idea for how fast your connection is (this will vary but shouldn't vary too much... what you're looking for is a big repeatable drop on downstream *and* upstream speeds after any changes).

Now that you've got your baseline, stick a garden variety splitter in the path and connect everything up. Go look at those numbers again. As long as you haven't gone below/above the mentioned levels, you are good to go. Run a few speedtests (I like dfw.speakeasy.net, which then allows you to use any of several tests around the country) and if you don't have any major changes in the quality of your connection, you needn't do anything else.

If the splitter DOES cause a problem, use a tap (proper name: Directional Coupler) instead. You'll have to order one of these, it's not something you'll find at a Radio Shack. It's basically a splitter that has a ~1dB loss on one output and a ~6dB loss on the other. They're for daisy chaining outlets together (as in a hotel), but also work for our purposes as the cable modem feed is almost lossless. For TV, you can compensate with an amp if it's snowy or you have digital dropouts. For cablemodems, you really can't. I use one and it works perfectly. Here is one source. Haven't used 'em, they're just what came up first in google!

One further issue. Amplifiers DO NOT help cable modems. Almost all only amplify the forward path and pass the return without amplification. They also add noise. So if you have an amp, it's best to put the modem ahead of the amplifier if possible. If you don't have enough signal prior to the amp, call the cable co. It's their issue and they need to fix it.