Testing if a random signal was modified or if it contains noise Given a random signal, is there a method of finding if it was modified or if it contains noise if you don't know the original signal?
E.g.: given a random image, can I find out if it was modified if I don't know the original?
No.  You have to start with some prior knowledge of the signal and of the noise.  Your signal may be my noise, and visa-versa.