I worked with junkies years ago for a few years and if its Cannabis the only sign might be the smell of Skunk which is a very powerful smell, it was mainly rocky in those days and Skunk is stronger than rocky. The Rizzlas, especially large Blue in those days, rolling machines, bongs, and people getting a sudden urgent urge to eat, the munchies were all signs. And of course red eye. Easily hold a job down.
With Crack its deep paranoia, scrambling about on the floor looking for minute specs of crack they think they have dropped. Volvic bottles, tin foil, elastic band as well but its the being all over the place mentally that was most apparent. Virtually impossible to hold a job down as their concentration is poor and they will be wanting a fix increasingly more often.
With Heroin obviously the needles and the marks not just on the arms, if the veins are fucked in the arms its legs, inside of groin, anus and neck. A Heroin addict with money can hold a job done, one guy did for years.
With Cocaine i could actually smell it in their pores, they reeked of it, obviously the sniffing. Being obnoxious and thinking they were the big i am. Being restless.
Speed and similar, could never sit still, literally buzzing and restless.
Then there are legal prescription drugs like Valium, myraid of anti-depressants that can have a massive effect on the mind and body.
I have met WGs who took all of these things between them, some more than one and as i am clean its really no fun for me, i am on a different planet to them.
If people want to take illegal drugs thats their business unless it affects me when it could be my business if i got nicked during a raid on a brothel for example, so any hint of illegal drugs and i would be off nowadays.