AAI Honda Civic Project Part 2, why would I check for a head gasket leak?

Part 2 of the Honda project is to discuss with new techs why we would do the compression test and possible head gasket replacement to come. In other word what would the customers concerns be that would lead me in that direction? If you are an experienced tech please leave in the comments any experiences or symptoms you have seen that led to a head gasket diagnosis, or better yet a time when a suspected head gasket leak turned out to be something else?