German phrase
Woran würden Sie ~?
Meaning
How would you tell if ___?
When to use it
It asks how to tell. It politely asks what evidence someone would use to decide.
Example dialogue
Woran würden Sie erkennen, ob das Produkt echt ist?
How would you tell if the product is genuine?
An der Verarbeitung und an der Seriennummer würde ich es erkennen.
I would tell by its workmanship and serial number.