As far as I know, there is know way to find out if a URL on some far away host exists. (At least not within ansi-c.) Far better, the braindamaged concepts of w3 do not support bidirectional features. Simply speaking that means you can point to something but don not know who is pointing to you.
Anyway, we will have to live with w3 (and bloody pseudo-bidirectional improvisations like annotations) like it seems that we have to live without a useful 24-bit file format and nearly never working german umlauts.
So, after all that pessimism, the positive part: hsc is able to check existence of local URLs in you html-project. At the moment, only the tags <A HREF< and <IMG SRC< support this feature. I currently don't know about any tags that support URLs, but I will have to see for a html2.0-docu.
When you refer to a non-existing local link within your source, hsc will display a warning message.
The URL-checking can be enabled using the ChkUrl switch.