How to find the public name of a contact on Viber?
It goes when you delete the contact off your local contact list and then you intend to add it thru your Viber(tested on iOS and Android). 

How I did it:


Fount a contact number that I wanted to see his/her name
Remover it off my local contact list
Opened Viber and Navigated to More, followed by Add Contact
Entered the contact number and pressed Done (on iOS)
Please note that it does not add the contact, nor it notifies the recipient anyhow of the resonance of it's public name.