>>803354it depends on how harsh their laughter is, their expression during the laughter, and how they look at you afterward.
If their laughter is friendly and short, facial expressions are mild, and are still chatting with you, they're laughing with you. If their laughter is harsh and exagerrated, facial expressions are extreme, and they're giving you a smug and condescending face, then they are laughing at you.
It also depends if you said a stupid thing as a joke or as a serious comment.