One some Vega series cards like RX Vega 56, RX Vega 64 and Vega Frontier edition you might see GPU Tach activity LEDs on the back of the card near power connectors. 

Idle graphics card would show only one LED, but when there is some load then more LEDs will light up.

You can control them with a DIP switch on the backplate.

Please note: On alternative Mac BIOS GPU Tach is not available.

They can be either red, blue or turned off completely.