I am a non-native English speaker, and you are not alone in getting confused.
However, we have two clues to help us get less confused:
- Look at the content
You should look at the expression in the content it is said. If it is used in negative/ positive content, it also probably means something negative/ positive.
E.g., That is ridiculous! He has no authority to threaten his students. (negative)
E.g., That is ridiculous! I can't believe she has passed all of her exams without even an hour of study. (positive-showing surprise)
- Listen to the tone
This is even more important than the first clue. Pay careful attention to how the speaker says the expression. Does it show sadness? Does it show surprise? Does it show praising?
Finally, don't be afraid if you couldn't understand the speaker's message. You can always politely ask, "Excuse me, I didn't get your point! What is ridiculous?"
Give yourself some time, and you will become more experienced over time.