Answer:
Nervousness, scratching, signs of skin irritation from scratching, small round sores (from skin-popping), localized swelling of veins, especially in the elbow area, unexplained sleepiness, appetite loss, or disinterest in food, dilated pupils (lots of black) in bright light, a sudden affinity for sunglasses, runny nose, frequent unexplained need to be alone, missing money and valuables, lack of attention to hygiene, increasing inability to function normally for prolonged periods, personality changes, change in friends, drop in grades, reduction in quantity and quality of work output, loss of enthusiasm for former pleasures....
No one thing is definitive, but these are some of the things to look for.