I assume you meant a resume on a Web page. Of course you can't track it with a DOC sent with the application or a resume on paper, for instance.

If you assign a unique but hard-to-guess "id" for each prospective employer that is embedded in the URL, you can identify if the page has been viewed by an employer by checking the access log.

Also, Apache's mod_rewrite can easily be configured to camouflage a URL like

http://blablabla/cv.pl?e=24f0a123be

into something like

http://blablabla/cv/24f0a123be

to make it a little bit more obscure to hide the fact that the ID is used for tracking purpose. Of course, smart employers who are familiar with Web will be aware of this sort of tricks and try to evade anyway, but that is something you may wish to try.