Separating two frequency component How can we separate two frequency component when they get mixed means when signal get mixed with noise how receiver can judge which part is signal and which is noise ?
You asked "how" without first asking "if", and, if so, under what circumstances.

If you add two completely unknown numbers A + B and get 100, can you un-mix the sum to the original two numbers A and B?

But if you know B = 7, you might have a solution.  

In the receiver case, if you know the 2 spectrums are disjoint (or other properties, such as redundancy in modulation/coding, etc.), judging might be possible.  Or the answer might be statistical.