How can apps recognize the device ?

I have an app the gives me a free trail but when i try to make another account to get the free trail again the app recognize my device .. i tired (deleting the app, changing the proxy, changing the device name, duplicating the app, "udid faker") .. nothing of these methods helped me to prevent the app from recognizing my iphone .. so, how can this app recognize my device ? please help .. thx in advanced .. :)