There is no easy command line tool for checking quotas. The best thing to do is log into your control panel and click "Site Summary"
You'll see a line that looks like this:
Bandwidth Monitor (for cycle beginning Mon Apr 01 2002)
Used / Threshold: 15 MB / 6000 MB (16626672 bytes / 6291456000
And a couple of lines that looks like this:
Disk Used/Allocated 65.9 MB / 250.0 MB
Max Users 40
That's the easiest way to tell.