command with no arguments will show all filesystems that are mounted. So, you could issue mount on its own, grep the output for say, "/media/usbdisk", and then count the number of lines in the output. 0 means it's not mounted, 1 means it is.
As a little extra, the && operator between commands actually means "return the result of the first command ANDed with the result from the second command"; but since its lazy, what it means in practice (especially if you discard its return) is that it will execute the second command only
if the first one returned successfully. You could thus have your cron job running "check_mounted && cp...", which would bypass the copying if the check_mounted command didn't return successfully.