dclone how to know

how can you tell when some one has

Diablo Clone in a game. Plus where can i find out what location he is in?

you will get a message in game
"diablo has invaded Sanctuary"
the screen will shake and go slightly darker.
then you get to "choose" where he will spawn. dclone will replace the first superunique monster that you encounter after the message is shown.
most people go to the

Frigid Highlands and walk up to eldrich, as this is a quick place to get to with few other monsters to fight.

ty