I usually flash them that it is clear to move over. Always look and see what/who might be pushing them impatiently trying to pass the truck as this will be when they will most likely be looking to move over earlier.
This is where you do the trucker and yourself a huge favor by giving them clearance at the first safe opportunity. As mentioned watch their mirrors and when they look over, usually just prior to activating their turn signal, this is the time to flash.
If there is no one close on the trucks rear end they can usually find their way back over safely whether you flash or not but it is still a nice, courteous thing to do.
:C