URL Forward them each to unique pages on an existing website. Analyze your log files.
Alias them to an existing website. Check your referrer logs for entries coming from that domain.
Create a specific website whose only purpose is to be a nexus point for all your unused/undeveloped domains to point to. Put a script on the default page that captures the domain (use PHP) that was typed in (maybe the HOSTNAME env variable) and sends you an email every single time someone types it in.