There's no single answer for that. The server we use is win2k so I can use the Remote desktop app and check whatever I want about the server. If you have a linux box there are various other ways to do the same thing. If you have a web-based control panel there is probably an area where it gives the system info so that you can have a looksee and see how much diskspace, etc you have. If you access your server via ssh there are ways to do it also but I don't recall the particular commands cuz I haven't needed that in a long time. If your host doesn't provide you with any of those 3 means of connecting to your server you can STILL use a variety of server info apps to check that info from the web.
Checkout
http://sourceforge.net/projects/phpsysinfo if you run linux and
http://sourceforge.net/projects/winsysinfo if you have a windows server. Both are php scripts that you can install via your ftp program and they report disk usage, processor, etc, etc, etc...
There ARE unscrupulous hosts floating around out there. One guy I know was paying for a 2 gig p4 and he was actually on a 300mhz p2. That's pretty extreme but it really is smart to check that you recieve what you pay for.