If you can or ought to be able to notice something, it's detectable. An odor in your fridge is detectable. Or, there can be a detectable coolness in someone's behavior toward you.
Detectable is related to the word detective, one of the guys on the police force who finds clues and tracks down bad guys. And that's a good way to remember detectable. If something is detectable, it can be seen, smelled, heard, felt or measured with a machine. Think about detectable levels of toxins in the water, detectable footprints in the snow, or detectable traces of blood on the carpet.