You can catch most failures before they hit your palate. Watch your brew: if the coffee drips through in under 2 minutes, you're likely under-extracting. If it takes over 4 minutes, you're over-extracting. The bed of grounds should look flat and even after brewing—if there are holes or craters, you had channeling.

Smell the grounds after you pour: they should smell sweet and fragrant. If they smell sour or burnt, adjust accordingly.