Are you concerned about phishing e-mails where people send links to ip addresses or incorrect domain names? If this is the case, just don't go to ip addresses or domains you aren't familiar with, and definitely don't give them any sensitive data.
Are you concerned about DNS spoofing where you go to the correct domain name, but it resolves to an IP address of a compromised system? There wouldn't be any way to detect if a site is being spoofed using an MD5 hash, because where would the hash come from, the web server which is already being spoofed? I don't think a site using an SSL certificate signed by a certificate authority can be spoofed, though.
If you want the md5sum of a page's html content
wget -q -O - http://ubuntuforums.org/|md5sum