For simplicity sake let us take the scale Mohanam. The scale would be
S R2 G3 P D2
The references to R, G and D mean the Chatusruti, Antara and Chatusruti respectively. Now in the mdoern classification Mohanam would become a janya of Harikambhoji as this the first melakarta to carry all the swaras that occur in Mohanam.
Now coming to the actual listener, when Mohanam is performed, he only listens to the melody that is Mohanam. A familiar melody that would remind him about Ninnukori or Mohana rama, Pazhaga theriya vendum or just some oriental melody depending upon his own level of understanding. This above familiarity is not enough to understand why Mohanam is a janya of Harikambhoji. The reason is that the classification is swara based and not melody based. However if the same listener has acquired the ability to distinguish swaras clearly when hearing he would know what the notes are that occur. For instance he would know that the Mohanam gandharam is G3 or its rishabham is R2. It is this ability to distinguish the swaras that helps the listener to understand the modern mela-janya classification scheme.
Of course given the technology and information that is available to us, the listener who can “live blog” a concert can also Google the ragam and check the notes that occur and find out what janyam it is!! Now here the listener is only accessing information and NOT understanding the information. The listeners also get this information over a period of time through sustained exposure to the music.
Sometime in the early eighties I was listening to a concert by TV Sankaranarayan, and he was singing the raga Natakapriya. I had no idea what the ragam was but because of my basic swara gnana acquired through 7/8 years of formal learning, I had fixed the scale and the notes that were occuring. My dad who was sitting next to me said it might be Natakapriya! Once the song Gita vadya started we knew the raga. I still could not understand how my father knew it to be Natakapriya. He said that sometime in the early seventies he heard a concert of TM Thiagarajan and after finishing the alapana TMT himself announced “This is Natakapriya. Todi below & Karaharapriya above!” Now here my dad was depending on identifying the ragam purely on a melodic basis without any swara gnana involved.
Anyway I am getting carried away a little but I want to make the point that if listeners have the ability to distinguish swaras clearly then it is obvious that they can identify the parent raga. They can also see if the raga sung is a complete 7 note scale or if any notes are missing. They can say if the scale is sampoornam or vakram. The thing is that this knowledge will be more than sufficient to identify the parent raga or even determine whether the ragam being performed is a mela or a janya in the first place!
That is why it does not really make sense for someone to aks what Janyam Shamalangi is if they could not make out that it was a mela in the first place.