If the tooth is already filled and you start having cold sensation or some thing like food starts sticking to it then one can say that te cavity has started in the tooth. it is called the secondary caries. To confrm this X-ray has to be taken and the tooth has to be examined clinically. Clinically there is a gap between the tooth and the restoration and it is blackish brown in color. For more info on dental caries you can visit the following link......http://www.identalhub.com/articlecategory-dental-caries-7.aspx