Directory verify ownership url of submission

So I want to use Directory plugin to allow users to submit business listing including a url to thier own site. But how can I verify ownership of the url that they submit?

I want to do something like ask them to place a file or some other readable image file into their own site, an action only a webmaster can undertake. Then I can verify them as the owner.

The last thing I want is a directory full of spammy links to trash website businesses.

Could I create a note that is visible to the user during sumission that says, “place this file…” and then moderate the listing submissions?

What do people think?