Taste buds can detect chemicals in as concentrations as low as a few parts per million, I dunno.

Someone once said the reason we had alcohol before civilization is that we carry around a chemical testing laboratory in our faces.

It just so happens that everything in beer that can go wrong and hurt you (any sooner than cancer) creates a distinct aftertaste and you can learn to avoid it rather easily.

The only exception of course is if you use poisonous ingredients in the first place.