What are signs of addiction? (or, how can you tell if someone has a problem with addiction?)

Sudden changes: Loss of appetite, increase in appetite, any changes in eating habits, unexplained weight loss or gain. Slowed or staggering walk; poor physical coordination. Inability to sleep, awake at unusual times, unusual laziness. Red, watery eyes; pupils larger or smaller than usual; blank stare. Cold, sweaty palms; shaking hands. Puffy face, blushing or paleness. Smell of substance on breath, body or cloth.

Change: In appearance like looking disheveled, or becoming disorganized, coming late when they are usually early etc.

Other signs : Be cautious about stealing and lying as well.