Partial list of behaviors and items to be aware of:

Common drug effects
Anxiety
Panic attacks
Depression
Memory loss
Hallucinations

Sedative effects (GHB, Ketamine)
Slow breathing
Confusion
Tremors
Nausea
Intoxication
Uncontrollable movements

Club drugs toys or paraphernalia (partial list)
Pacifier (sometimes candy - to prevent grinding of teeth)
Mentholated rub to inhale for heightened sensory experience
Dust mask or tampon tubes for inhaling mentholated rub
Light sticks, glow jewelry
Large amounts of bottled water or empty water bottles

Club drug language (partial list)
Rave - Large gathering intended for dancing to music with light displays Raves or "alcohol-free" night clubs can be a platform for club drug sales and distribution
Rolling - What being high on Ecstasy is called
Cuddle puddles - A group of "ravers" giving back-rubs
Trip - An acid or LSD high