Being able to unscramble a complex signal is simply a matter of understanding the probabilities of what each signal might represent for a particular type of data stream. Also, signals transmitted in return to zero format look different then those sent non-return to zero. Modulation also is a problem.
Signals can modulate on a carrier or appear as a baseband source.
Amplitude modulated signals are the easiest to identify, and also the simplest to demodulate. Even with a trained eye however, since background noise sources are usually high, and since Gaussian noise distributions can sometimes look like signals, simply assuming that a detected signal is a compromising emanation can lead to massive overkill in a redesign effort. The bottom line is that in the majority of cases, an engineering approach that disregards communication theory, and simply addresses redesign through trial and error test techniques, is both expensive and difficult to achieve in a reasonable time period. The approach usually results in long redesign efforts, and often simply shifts the problem to another frequency or location, which is then found at a later date resulting in additional problems. What they don't understand is that signals radiate because transmission line theory tells us that if the signal channel's source and load impedance don't match, which they never do at all harmonics of a signal, then standing waves are created and some energy escapes to the outside world.