From Hermetic Systems
Hermetic File Monitor is for detecting changes to files in a folder, and for detecting additions or deletions of files in a folder. Multiple folders can be set up to be monitored (and there is no limit on their number). Optionally files in all subfolders of a folder can also be monitored. Optionally all types of files can be monitored or just files with a certain file extension such as .xls. Folders can be checked either manually or automatically at regular intervals. Results can be written to report files.
Hermetic File Monitor works by recording MD5 digital signatures of files (using the MD5 message digest algorithm). The MD5 digital signature of a file is a sequence of 16 bytes, usually displayed as 16 hexadecimal numbers with no intervening spaces, for example, 4EAE15241AD90F85D038523E9D14DDDB. If the digital signature of a certain file is the same at two successive recordings then the file has not changed. If the digital signature changes then that shows that the contents of the file have changed. Files with unchanged digital signatures have not been modified. Any change in the digital signature of a file shows that that file has been modified.
Comparing digital signatures of files is more reliable than simply checking their date/time stamps (as some file monitoring programs do) because the date/time stamp of a file can be set so as to disguise the fact that a file has been modified.