How do you know the synonym of a word in a sentence?

Normally you would consult a thesaurus.

In Microsoft Word, type the Word in, and right-click, you will see an option called synonyms. Other Word processing programs have similar definition features.

On WikiAnswers and Answers.com

Double-click on any Word in these paragraphs to bring up 1-Click Answers, which is a linked dictionary and thesaurus.