I can think of 2 ways, depending on what you want to do. If you want to check if it has been modified since it was written to hard drive, simply compare the file creation date with the last modified date (new to win 95/98/me)
Or you could do it like Microsoft Anti-virus did. Save a checksum for each file in that directory in a file called checksum.vb or whatever. Compute the checksums again on a periodic basis and compare them to the old ones.