how to tell if bass is good on a set of speakers?

Yes its the frequency response.

Frequency response is given as like 10 Hz to 20,000 Hz (20 KHz). Some right the second value in Hz and some in KHz, so don't get confused.

See the first value. It should be as less as possible. A speaker having 5 Hz-20,000 Hz have more BASS than the speaker having 10 Hz-20,000 Hz. And the second value indicates the treble. The more is the second value, more the treble is.

Bass is the lower frequencies of a sound. So the lower the frequency, the greater is the Bass.

I suggest Creative's Gigaworks series 2.1 speaker

Look for flat frequency response down to about 70 or 80 Hz.

Few affordable speakers can do any better than that.

Few 2.1 systems go much lower than 100 Hz,

even if their specifications say otherwise.

A really good (separately-purchased) subwoofer can reach down to 30 Hz or so;

the very best will reproduce frequencies so low that

the vibrations are physically felt rather than heard.

These cost enough that you probably won't be buying one anytime soon.

The best way to find out what a speaker can do is audition it in person,

but there aren't many stores where you can listen to "computer" speakers

at all, let alone in a realistic (homelike) environment.

Lance is correct. You can make some judgement by looking at the frequency response but it should be a statement of such within + - 3dB. However even a good, low cutoff frequency does not indicate quality. That woofer's performance may have poor transient response and considerable distortion, like the 3rd order bandpass bass modules sold by a certain major manufacturer. Reading reviews - unbiased reviews, not ones published by the manufacturer, can be of value.

Also, there is a lot more to speakers than just bass.

The only surefire method is to listen to them.