"Split" v "Composite" can be difficult to determine, especially if the AOB is fine, and the secondary is near the primary superstructure. Masts in turn are somewhat range-dependant, ie can be hard to use as identification at great range. With practice, you'll find yourself recognising the most appropriate features, to use for any particular range/light level/AOB combination.
There's no "magic-bullet" or single rule that "solves this", it comes with lots of practice. If you play multiplayer, then ask the captain for practice in IDing ships, so you can ID the ship, ask him to "confirm the ship MMF at bearing xyz" and them see if you agree. With practice you'll find the quickest and most accurate indicators.
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