How can you tell if something is a good fat or a bad fat?

Good fats are polyunsaturated and monounsaturated fats, as opposed to saturated fats. Likewise, "good" lipoprotein is HDL and "bad" lipoprotein is LDL.

An example of a "good" (heart-healthy) fat would be olive oil. An example of a "bad" (artery-clogging) fat would be margarine.