Which way Way Marker?

I do love way markers.
Spotted this one while sitting at temporary traffic lights on the A5.

Apparently it's a replica milestone of a Thomas Telford plate.
I love the way they've had to chip away to get the word Llangollen in!
(not sure what dad - a stonemason - would have said about that?!)

So the question is - how does one know which direction is which??