When signal is received how can receiver know which part of received signal is signal and which part 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.
I agree with Marcus Mueller, the receiver doesn't know. It uses a decision rule that was designed to minimize the probability that an error occurred in the received message given known and assumed characteristics of the signals that transport the message and the known and assumed characteristics of the noise. Noise can be many things, including other interacting transmitters and receivers.
The receiver doesn't know. It makes a "best" guess. The transmitter doesn't know if the message was received either. In many circumstances, feedback can improve the probability that messages have low probability of error.
There are other kinds of channels outside of this model such as the erasure channel where messages are lost between transmitter and receiver.
It is proven that all noise combined will form a distribution of gaussian function hence most of the receivers are designed with what you can call a matched filter what it does is reverse and shift by bit interval(tb) and integrate it & sample at which it has highest snr (of course it is at tb) but for real time application we use threshold detection and fec(forward error correction) and digital system we even have repeaters that placed bw tx &rx to minimize noise and if you use ofc it not effected by external noise so I suggest to read at least the basic block diagram of digital communication
Your question is too general, you need to specify a few and ask a targeted question but this doesn't mean that your question isn't good, so let me give you an answer to your question. You question concern the signal postprocessing and processing parts of a telecommunication system (transmitter and receiver). In the receiver channel, we found many components like LNA(Low Noise Amplifier) and then we find filtering operation to keep the desired frequencies. Pulsed radar, for example, use the matched filter (correlation) to detect the targeted signal in a noise Hope I gave you a good answer Good luck