If a domain shows a generic landing page like this one http://www.domainlandingpage.com/ then you know it is not being used but people are just keeping it so they can sell it to someone who actually wants to use it. I guess the question would then be: how can you detect if a domain is just being parked.
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It might be being used for email rather than website hosting. I have a few websites with only MX records, and one domain registered which only has a bunch of SRV records in DNS.
You could try checking if the nameservers assigned to the site are operated by the domain's registrar and if so if they are used primarily for that registrar's parked domains.