I don’t know whether this is the best way, but it seems to work:
A room can be good or bad.
The Kitchen is a room. The Kitchen is good.
The Playroom is south of the Kitchen. The Playroom is bad.
["Before" line taken from Recipe 213 Eddystone]
Before going from a room (called source) to a room (called destination):
if the destination is bad:
say "You enter what seems to be a bad room..."
You enter what seems to be a bad room...