Like attracts like. If you are an experienced drug addict, you will in most cases (note: I said most), be able to spot another person with the same poison.
I've had experiences with all sorts of drugs—let me tell you, I can spot an opiate addict or tweaker from a mile away! In many cases, they aren't even necessarily blatantly using. Drug addicts are not always (but often are) unhygienic and either on the nod...or 'wild eyed'. I've always been an avid people watcher, and I can observe the way a person carries themselves and quite often accurately determine (and later find out) that they were/are an addict.
Sometimes the defining moment can be something as small as the words they use while talking. Drug addicts often have a different set of lingo due to the 'street smarts' we are forced to obtain if we want to things run smoothly with dealers, users, ect. We also have a different set of mannerisms.
A lot of both practicing and non-practicing drug addicts have a set of 'cancelled eyes'...eyes that have a sense of guilt, pain, and doubt behind them. Drug addicts also often tend to either make too much or too little eye contact. After getting 'caught', we begin fooling the people we're able to by staring them directly in the eyes. Also, there's that looking at the floor/around the room move. I see that one a lot with active AND non-active tweakers.
Last but not least, the obvious—that physical appearance that says it all. That "rode hard and put up wet" appearance. Not all drug addicts get this, many are in denial that they have it, but it is frequently visible to another addict (even just a trace of it to me gives it away)!
I'm going to throw in a personal example here to perhaps elaborate. From what I've been told, I have luckily been pretty blessed when it comes to looking 'youthful' (for the amount of drugs I've consumed in the past 7 and a half years. Let's face it, though. We all have our moments. Haha.
Well, a few months ago I went completely overboard on Oxycontin and Lortabs. For whatever reason, Oxycontin always seems to add years on me quicker than any other drug...including heroin and meth. Anyway, to cut to the chase, during the height of this AM-PM opiate binge, I got summoned for jury duty. When I walked up the stairs to the courthouse, this young yet 'rough around the edges' looking woman approached me with a mischievious yet worn out grin on her face. I had a few minutes to kill, I was high, and she seemed interesting, so I sat on the bench for a bit and talked to her. As I had suspected, she had just been released, haha.
As the story continues, I let her use my cellphone and we joked around a bit. A few minutes later, I looked at the time and saw that I had to enter the courthouse for jury duty. I told the young woman goodbye and we exchanged mischievious grins once more.
When I got up and started heading towards the courthouse, she screamed "Good luck!" and gave me a bit of a wink and a chuckle. I responded with the same exact gesture and remark...only my chuckle was a bit more intense. I had gone there to WITNESS a trial! I wasn't ON trial! Haha. It was then that I realized I had better slow down on the drugs, hahaha. 8) Granted, the young woman
was pretty rad, but she had that wear and tear on her face that screamed "SEARCH ME, OFFICER!". After that experience, I literally detoxed cold turkey. When I do use now, I make it a point to look myself in the mirror and examine how different I look. If I look in the mirror and see that I gained 4 years in 4 days—I quit (if applicable), or seriously slow down until I look less gaunt in the face.
I'm a bit too narcissistic to turn into the female equivalent of Keith Richards.

Anyway, just my two cents (possibly four since I tend to ramble). Hopefully, my words and experience helped answer your question at least somewhat. :D