You can't kbow except to guess from context, but if your Korean was good enough to be able to do that, you wouldn't be using Romanization in the first place. Questions like this are part of the whole point - Romanization is useless, except for maybe the very very beginning of one's Korean studies. One language's letters could not possible reflect another language's sounds. The Korean alphabet is an integral part of the language that should be learned as soon as possible. Now, if you're looking at Romanization for a certain reason (perhaps putting song lyrics into Hangeul), then good luck! You'll have to use context. Plus, there's no standard way of romanizing. I believe three systems are exist for Korean.

There are no one-to-one mapping between Korean character and romanization. You cannot know the original Korean characters from the romanization of a word. You should make inferences from the whole context of the sentence.

I'd like to add a bit more detailed explanation. you can only choose the correct Hangul consonant when you know (or at least can guess) the meaning of the word and/or the grammar structure which is used. for example: 맞는 (mat-nyn, =correct) derives from adverb steam 맞 + suffix (or ending) 는, so we write ㅈ as in the original adverb steam. 했던 (haet-ton, =used to be done) derives from the verb steam 하 + suffix 여 + past suffix ㅆ + past suffix 던, so we write ㅆ because the corresponding past suffix can be written like this only.