The way someone talks about the absent person tells you what absence may cost you later.

How a person handles someone who is not in the room is a fairly reliable preview. If the absent colleague is described with a little more contempt than the present one would receive, you have learned the terms of the friendship: you are safe while you are here.

Notice, without announcing your conclusion, how the people around you speak about those who are not there. Then watch what you do yourself, because this one is easy to join in with. Declining to add to it, quietly and without a speech, is usually enough to change how a conversation goes.