URL monitor - alert only if available

We have a web content filter that I'd like to ensure is blocking inappropriate websites, so I'd like to configure a URL monitor to only alert if a given URL is available. Is there any way to achieve this?

thanks!

Also - the OpManager Knowledge Base found at http://support.opmanager.com/sd/SolutionsHome.sd is broken and has been for over a week. Attempting to do a search returns:

HTTP Status 404 - /sd/showTopicDetails.do
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type Status report
message /sd/showTopicDetails.do
description The requested resource (/sd/showTopicDetails.do) is not available.
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Apache Tomcat/5.0.28