Here's a different twist, I'm a recovering addict/alcoholic, drug of choice, opiates.
I have nearly three years in Recovery and if you want the scoop of what to look for, I can describe myself quite clearly when I was using.
But first. If the person you are asking this question about has a history, exactly what do you expect to accomplish by knowing what to look for? I am not being sarcastic please don't think that. Honestly though, is it going to help you at all knowing the truth? Like rahsue said, knowing all of the signs isn't going to help the addict at all. You can educate yourself on every aspect of addict and, unless the addict wants help, it doesn't do one bit of good. I know when I was high and someone close to me called me on it, I denied it up and down. I either got really mad or would act all hurt,"How could you accuse me of that? I can't believe you! You don't trust me at all, do you? . . . " (Pour on the tears here) Someone could actually see me swallowing hand fulls of pills and I could stand there and say that they must be seeing things, I didn't take a bunch of pills! I was taking some tylenol, what's wrong with you! I'd try to make the other person think they were nuts.
The biggest sign when I was using was hyperactivity. I know almost every addict who's drug of choice was opiates get hyped up too on them. The "normal person" takes pain pills and it knocks them out. Seems like addicts who develop the love for these pills get hyped up. My ex husband, when we were married, could come home from work and before he even saw me, he looked around and saw the house was spotless, the laundry was all done, there was a wonderful smelling dinner cooking, a dessert was cooling and I was just sooooo happy to see him. Also, I was very, uh, anxious to get to bed, if ya know what I mean. And adventurous too. Once the lovin' was over, I would kiss him goodnight and explain that I just wasn't tired and was going out in the living room so he could sleep. Opiates had me speeding basically. Some nights I didn't get to sleep until the wee hours of the morning.. Then I was a real witch the next day cause the pills were usually gone.
I also did a great deal of scratching, mostly on my nose and face. I call this the narcotic itch. "My nose just itches, I feel like I have a hair on my face that I just can't get." I even went as far as to blame it on new makeup, or soap . . . laundry detergent. You name it.
Just remember, an addict will deny using til the end of time if they don't want help. And also, you have done nothing to cause this. No matter what the person says. None of this is your fault. We addicts are very good liars, and we can turn the finger pointing on you in a heartbeat, try to blame you for us using. You are not to blame.
Take care of you,
Judy