I found waving helped, as it's more obvious.
If it's a straight nod they could just be looking down at their clocks, if it's a side tip that could be checking a wing mirror.
If they're bouncing up and down, waving a hand and giving thumbs up, it's pretty clear!!
If they're flapping the right hand up and down on an extended arm, as if they're patting a gnome on the head, slow down - There's likely a hazard around the corner that they're warning you about.
If they point up and to the side, that's Speed Camera Ahead.
If they put a fist atop their head (or similar) that means there's a cop ahead.