I have been on the following in front of people, at work, with close friends or loved ones, have driven, and functioned at work or in school without anyone noticing a thing. What matters is dosage, tolerance and knowledge of how the drug effects you. NOT any specific compound.
I do NOT recommend doing any of this shit. I'm just going to say that once you use any (ANY) drug enough and in low to high doses, they are all very predictable.
benzos
heroin
cocaine
meth
25C-NBoMe
2C-E, 2C-C, 2C-*
MDPV
MDMA
Methylone
Alcohol
Nicotine
Caffeine
Aspirin
The only "noticeable" thing people will see is your own unexpected reaction to a drug (or anything else.)
Taking a lot of Xanax or drinking a lot will slur your speech. There's just no way to hide that.
I have lost my car a few times and people just thought my memory sucked. The ones that knew I use drugs assumed it was a benzo. The ones that knew me well assumed Xanax because I'm personally more likely to binge on this specific drug that hinders memory.
I have zero recollection of some of my days and nights on Ambien and Phenazepam (not mixed). I apparently acted very normal and even got a handful of girl's numbers and spoke to cops with no issues.
There's no way anyone can detect you're on something because the majority of people will never assume anyone else is on drugs. You'd have to be experienced to pick that shit up. I can detect drugs very accurately if I have a baseline to compare the person to. For example, I know exactly when MDMA kicks in because my friend would never say something this nice to this person. I know a person is tripping if their sense of time is off or if they notice miniscule details. For example, when I am tripping very hard, I know from experience that I'm experiencing time as dilated and that I'm taking in a lot of sensory information. Therefore, while walking past people at normal pace, I can detect something they are wearing, something specific like a piece of an earing or a small facial mark, and I can comment on it to that person in real time. In "reality" a sober person would have some things on their mind and so be too busy to care about details like this around them.
Obviously I could be completely wrong. I noticed that some very spiritual people have a high enough sense of awareness that I can't differentiate it from being sober or being on acid. Practicing monks, specifically.
People who play music can pick out sounds much much better than those who can't. Pilots have very sharp vision. Some people just have wit. These can all be mistaken for drug use and vice versa.