If I know of a physical hazard ahead I will warn everyone I see ("A rock slide blocked the trail. Sweetwater Spring is dry, next water is a jillion miles," etc). If I think that someone is badly underequipped I would try the BS line of warning ("You are a lot tougher than I am, heading out into the Mojave Desert in Aug with only one can of beer and no water"). Just starting off with "dude, you don't have enough gear" will usually get your glared at, or worse...