Only way you can do that is if you host it (or a portion of it) on your servers...although customers typcally don't like that.
You can also ask them to sign and fax/snailmail back a purchase contract/End Users Licence Agreement, that forces them legally not to sell or re-distribute copies of the software. For tracking purposes, you may want to include a function and hide it in the software so you can check which serial number is being used without having access to the remote server. If you see multiple copies of your software, sue the company who's redistributing.
Regards,
Matthew Madigan
Structure and Design Systems
a Division of Imadigan Holdings, LLC.
mmadigan@structureanddesign.com