Well, this is even more simple, you describe and or name their dog or cat, then it's perfectly clear who you are talking about!
On a walk I will notice and recognise the dog first, then I'll look carefully to see which human they have with them.
On the road it's a case of spot the car/recognise part of the registration number, then peer into the semi darkness to see who's driving it and wave. Or, brake and pull over because there is a huge tractor coming, then wave on the basis that it might be so-and-so inside, but it's too high up to actually see for certain.
The other trick is to stop in the middle of the road, open the window and you both have a catch up chat as the driver's seats are conveniently placed so you can do this. I do this one when someone has moved and it's the first time I have seen them for a while.