Recently I bought system from vendor with RX Vega card but I want to confirm whether it is Vega 64 or 56, can someone let me know how to check ?
Download GPU-Z.
Assuming the vendor is honest and didn't mess with the card's BIOS the easiest way is to open AMD Radeon Settings, select the System tab and then the Hardware tab.
GPU Z and Adrenalin from a Vega 64 Air (Black).
Look at compute units (ROPs in GPUz) 64 vs 56, texture mapping units 256 vs 224 and shaders 4096 vs 3584