How do you know if it is safe to remove a usb

[Linda]

I have a script  that uses cp -a to create a backup to a 
mounted usb volume.  In the past I have used Ubuntu and 
nautilus. In Nautilus when you unmount the usb drive after 
finishing the script it gives you a warning that it is 
writing to disk and it is not safe to remove the usb drive. 
In PCManFM does not give this message so in PCManFM how do 
you know when it is done writing the content that was cached 
and is actually safe to remove the usb drive?  Is it when 
the drive disappears from the file list?
                            Thanks
                             Linda