Unfortunately using slang is a tricky matter. Context is crucial. Once, when I was visiting elderly American relatives, we were playing bridge together, when, looking at my hand, I commented: "What a pile of crap!" - A stunned, dismayed silence followed. Oops!
It's particularly difficult because slang is used a lot more in French than in English. To viz, the infamous: "Casse toi, pauv' con!" At faculty meetings at Brown, I virtually never hear slang. But at faculty meetings in France, well-chosen (not all) slang words are a natural part of the vocabulary.
So how can I know when slang is or is not appropriate, and what is the scope of a particular word that I am itching to try out? The answer is: the online slang dictionary. It gives not only the meaning of a word, but also its degree of "vulgarity". Very useful! That might save me from other embarrassments in the future. (example.)
From that website, after certifying that you are over 18 years old (!?), you can even access a list of the 100 most vulgar slang words. I was able to check that I do not use any of them, and in fact, there are many whose meaning I do not know. I love the internet!