I assume you mean that you want to check when a new file is put into a directory. I had to do something like this once. Here's what I did:
Using java.io.File.listFiles() I got an array of files in the directory. And then after a period of time, something like 5 secs which is configurable, I used some set operations to determine if any new files were put in the directory.
Unfortunately I do not have any of the code anymore, but here's something: