Several... Signs of active tooth decay are: discoloration of the affected tooth, sensitivity to hot or cold and/or sweets. Some teeth don't display symptoms, that is why the dentist takes radiographs to determine carious activity in areas that are hidden and non-symptomatic. See your dentist regularly, get your teeth cleaned and don't worry.
Pain. Pain upon drinking cold water or eating something sweetened with sugar will usually cause pain in a decayed tooth. However, having exposed tooth roots due to gum recession can produce the same symptoms, even though there is no decay there. A dental exam is needed to differentiate between the two.