If it's a remote FTP server that you wish to check for files, and your FTP task is in the SSIS package ... then you won't be able to check for files without running the SSIS package. If you check for files in an initial FTP task within the package, then you're firing up an SSIS package every 15 minutes, which probably isn't a good practise.
The next thing that happens is that the business asks why it can't run every five minutes, and then they ask why it can't run every two minutes. It's a lot of server overhead.
On occasion I have used two SSIS packages, one to perform the FTP transfer, and the other to do local processing, and set these up as two separate steps of a SQL Server Agent job. The reason for that was to separate the error behavior. But it was for a nightly job, not something that ran every 15 minutes.
If you're checking for files on the local network, or using FTP directly, then you could use a batch file or something as the first step of a SQL Server Agent job (and abort if necessary). That would keep the overhead lower.
batch file
to check if the files are present in ftp else use ascript task
in SSIS to check for files. If files are present then proceed with package execution else abort – praveen