Each step in a web monitor can check for an HTTP status code. There are a number of valid status codes that you may want to create tests for though in general use you will probably come across status 200, status 0 and status 304.
Status code 200:
This means OK. If your receive this you can assume the page was presented successfully.
Status code 304:
No modification: This means that the page has not been modified it was last requested.
Status code 0:
This usually happens if the request times out. Try increasing the timeout value slightly in the web monitor step.
A good list of status codes can be found on Wikipedia here
