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.