How Can Telecommunication System Know Which Part Is Signal and Which Is Noise?
To elaborate on MM's answer just a little bit.  Unless the receiver has some sort of expectation of the nature of the signal there is no way to tell.  With an expectation (for instance a pure tone), only a best estimation can be made with the rest assumed to be noise.  With something like serial communication a voltage past a threshold is considered a "1" and no (or low) voltage is considered a "0".  In noisy channels, parity bits (or error correcting codes) are used to ensure "what was sent".  There are many possibilities, but the receiver has to know the nature of is being sent, which seems to be the answer you are fishing for.