Ultimately, no.
Presumably they have to be able to access your pages somehow - otherwise they'd be just like the other six billion people out there and you'd be paying them the same way.
If they're determined enough, they can fake a "legitimate" request for a page and you wouldn't be able to tell the difference.
Unless there is something like a "challenge/response" step involved - where something that (given today's technology) would need either some genuine intelligence on the part of whoever is at the other end, or such a huge supercomputer expense account that they wouldn't be interested in "catching pages".
What such a test would consist of (or whether it could be worked in to the structure of your site) depends strongly on what your site is and how it pays people for what.
