Hasty generalization, if I'm correct, is synonymous to "jumping to conclusions." So you could say something that has to relate to jumping to conclusions and not judging a book by its cover. For instance...
"As her sky-high heels clicked against the pavement, I watched her shiver against the overbearing chill in her unfitting cocktail dress. There was no way she was off to someplace morally just."
Obviously, the speaker made some prejudices about the girl because of her short dress and heels. Should you choose to use this example, you could elaborate on how the girl could have been going home from a party, or couldn't catch a cab, or maybe the dress was the only clean piece of clothing she had left. Things like that.
