SQL 2005 Integrated Services - DTS Package that only runs if source file has been modified

I have a package that imports data from a comma delimited text file into an SQL 2005 table every 10 minutes.  Currently, I have it set to every 10 minutes throughout the day because I need the data to be up to date and I don't know when the souce file is FTPd (It gets FTPd every few minutes sometimes but not at all hours of the day).  Is it possible to have the package only import and run if the source file has been modified (ie. FTP'd recently or content changed).  I believe that this type of functionality does exist for XML files, but not sure about comma delimited or how I would go abouts checking for this.  Thanks!  Any precise instructions would be greatly appreciated.