DROID identifies files using the magic numbers approach.
JHOVE is an extensible, Java-based tool developed for the JSTOR/Harvard Object Validation Environment (http://www.jstor.org/) aimed at validating digital objects. It builds on a magic number approach and adds much richer parsing functionality in order to extract more information and more thoroughly assess validity.
Inferno is a Java-based tool for rule inference and application of such sets of rules. It is currently heavily biased towards charcterization. It was developed at the Danish Royal Library as a proof-of-concept for using rule inference as an unifying approach to characterization, both directly to perform characterization tasks, and indirectly to integrate results from various existing characterization tools in an optimal way.
Inferno has been successfully applied to file and text string characterization by text file encoding (Latin 1, UTF-8, UTF-16LE and UTF-16BE), file type (text file encoding, JPEG and PNG) and language used in text file (Danish, Swedish, English and Norwegian).