I agree with thirteen, that showing a site in an iframe is the sure easiest way to replicate any web-site.
But since Google does not look inside iframes (nothing will be indexed), there is little reason for anyone to replicate a full web-site that way, if they do not at the same time show any other content around it that might make them money.
Still. It is as easy as checking the logs. if all requests seem normal, and all access stem from normal users, then it is not a proxy like situation, and an iframe is likely. Although you would then have to figure out why anyone would want to spend money iframing content that will not gain them any content credit with Google or anyone else.