The basis of composing is innovation and creation. Just as in literature when you are writing a composition, you should not use strings of words the way anyone has before or you might be branded a hack, or the likes. Instead you must script in original ways. Such is the basis of innovation and creation of which composing in itself is all about.

If you accept this than you can rule that composers who, for instance, can create an original melody, but have nothing new to say orchestrationally, by logic, are not truly composing, as all they are really doing is ORCHESTRATING their original melodies, using pre-existing orchestration techniques none-the-less. In this scenario, they would be an original melody writer and perhaps a very good orchestrator yes, but that is not enough to be called a composer.

Just as every element in prose and literature must be original, outside of the use of the alphabet, grammer, and words, and so every element in composing music should also be original and non-derivative outside of the notes, theory, and performance, respectively. In the end, a real composer must merge and simultaneously wield the power of Orchestration & Orchestral Device, Pitch, Melody, Structure, Harmony, Rhythm, and Feeling such that they are each motivated by the other and no one can be seperated from each other without the one, in isolation, being destructed to undecipherable form ... each one dependant upon the other and collapsing at another's dissappearance.

When comparing music writing to writing literature an interesting missing dimension is revealed between the two. In literature there is no direct element of color as there is in music. On top of the language of music describing and expressing in a way that makes a person feel a certain way, as would reading a poem or story, there is the element of instrumentation and orchestration that has no parallel in writing literature. The best analogy is if you were to write different letters, words, sentences or paragraphs with different colored inks and in different shadings. Now if you were reading a wonderfully original poem and every succeeding word were colored in the next color in the rainbow, you might think something is off. Where the writer was very original with their words and story (melody in music), they seem to lack the ability to be innovative with the use of color (orchestration in music). At this point, if it was acceptable in this analogy that color was an inherent part of literature, you might decide to move on to someone else's work seeing quite blatantly that this person was not entirely original with the medium.

So, you can see that if you are conscious of the degree of innovation that each element in a piece of music has, that you can more easily see right through a composer. In fact, as you develop this technique of listening for all-round innovation you might be surprised to see that a composer of this caliber is quite rare. You will also begin to notice certain nuances and creativity decreases in each individual element that can help you see where a composer's attention might have shifted to another element, forgetting to also pay close attention to the originality of the first element at hand. Honestly this can happen to anyone, and is forgiveable. As John Corigliano (Altered States, The Red Violin) said,

"Composing is the most difficult thing to do in life."

If you subscribe to the theory of the "basis of composing" discussed here than you can see how true that statement is over all other things creative on earth. Even over filmmaking, which is still in it's infancy.

This logic shows how important it is that a true composer actually incorporate idiosyncratic instrumental devices into the basis of their compositions. Bernard Herrmann and Igor Stravinsky are probably the best cases in point. For instance, Herrmann's famous cue from the score to "Psycho", which most of you ought to know and be able to hear in your head at this moment, simultaneously uses:

Orchestral device - Arco & Pizz Strings
Pitch - high pitch notes
Rhythm - repetition in straight rythmic succession
Harmony - extended minor 9ths and 7ths in broad stretches
Structure - cascading entrances going deeper and deeper
Feeling - intense vigorous playing in a straight deliberate tempo
Melody - fragmented motif in various pitch placements and contortions

Now, not one of these alone or even some of these in combination with each other creates that cue from the music of "Psycho". Go ahead and read one element above and decide if it is enough to describe the exact sound of Psycho. Of course not. There are many pieces that use "cascading entrances going deeper and deeper" in a different way that never eludes to the Psycho sound or music, many pieces that use "repetition in straight rythmic succession" in a different way, etc...

The best example of whether a film composer is truly composing non-derivatively, on every level, is if the music can be played in a simplified form and still remain presentable (not just recognizeable). If one element mentioned above is removed, the music should not continue to be presentable. Maybe recognizeable, but it should not be presentable in, say, a concert setting, without it sounding off base or incongruent with the original. Enter, the PIANO. If you can play a certain score on PIANO and it is presentable, then it is a dead giveaway that the composer is not truly composing, but perhaps is only a melody writer who orchestrates or some other combination of the elements aforementioned. Writing a great melody that is original and then applying the orchestrations from, for instance, Gustav Holst's "The Planets", is not enough to be really considered a composer. Because again, composing is about innovation on every level and that would be derivative in one respect. That is why a true composer is someone who is entirely original, or at least the majority of the time in motivation and execution. Gustav Holst's orchestrations were his original as were the reasons he did them in the first place. They were not to make a melody or set of melodies or lines that he wrote to come out on the orchestra but rather the lines he imagined couldn't be concieved without the orchestra being there in the first place.

It is the opinion of serious composers who subscribe to this logical philosophy of the "essence of composing" that concur that certain film composers are truly composing and others are not. Black and white. It is a strong orthodox view of composing, but it is really needed if you are to give any respect to those composers who standout as truly innovative and thus truly genius.

Here's some that fit the list through at least more than 50% qualifying scores:

TRUE COMPOSERS:
Bernard Herrmann
Jerry Goldsmith
Elliot Goldienthal
Howard Shore
Ennio Morricone
Dmitri Tiomkin
Dmitri Shostakovich
Sergei Prokofiev
James Newton Howard
Thomas Newman

NOT TRULY COMPOSING AS IT IS DEFINED:
John Williams (Despite some valid scores)
Miklos Rozsa
Bruce Broughton
Randy Eidelman

BORDERLINE:
Danny Elfman
Randy Newman
Marc Shaiman
James Horner
Alan Silvestri
David Newman