How to find the exact distance between the real world object and AR Camera?

I want to place the GameObject above the real-world object (for. eg. smoke effect over a coffee cup). how to achieve this?

 

anchor a gameobject onto your real world object and add the smoke as a child to that gameobject. Raise the smokes position Y value as you please.

 

how to know the exact depth of the real-world object from the ar camera?
I need real world co-ordinate to create the anchor right?

 

Create an anchor on whichever plane you detect using your AR. Calculate distance of the spawned anchor transform position and the cameras transform position.