There’s a couple different chemical compounds that can activate your bitter receptors (caffeine for instance) but there are a few bitter chemicals like Phenylthiocarbamide (PTC) or N-Propylthiouracil (PROP) that have been linked to variations in a specific gene (TAS2R38). Where I used to work we used PROP strips you’d just put on your tongue to categorize participants into tasters, non-tasters, and potential super-tasters. If it tasted like nothing, you didn’t have the genetic variant. If it tasted like bitterness, it could either be not that bad or, for example in my experience, it’s really really terrible and takes some effort to remove from the tongue.