How do you know an eggplant is bitter?

An eggplant can taste bitter if it has high levels of solanine, a naturally occurring compound. To determine bitterness, you can taste a small piece of the flesh; if it has a pronounced bitter flavor, it may be best to avoid it. Additionally, larger, older eggplants tend to be more bitter, while smaller, younger ones are generally sweeter. Peeling and salting the eggplant before cooking can help reduce bitterness as well.