The only way to tell whether 'dismal', 'gloomy' or 'morose' is the correct word if a dictionary doesn't help, is either through experience or through asking a native speaker. Obviously you can't do the latter on all occasions.
You ask where to find these words. You can find 'morose' in a thesaurus, under 'sad'. Then, you must check a dictionary for 'morose'.
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