I suppose it depends on what you mean by accessed. Do you mean read with a web browser? If so you can monitor the access log generated by your web server. Check Apache::Traffic for this.

If you mean something else, you'll have to be more specific. It might be very difficult to measure in any other context, since there are so many ways to access a file.

Scott

Update about Apache::Traffic: I suspect this will not be useful for you, as it appears to only deal with users, not specific files. Nevertheless, it should be quite easy to build a simple parser that parses the access.log file for a given file.