From personal experience, most people who can notice something different about you is because they have experience with the narcotic in accusation.
This. Everyone who has ever figured out I'm a pothead when I've been trying not to let it show, has been someone who either is or used to be a toker themselves.
Don't underestimate how easy alcohol is to smell. Someone who's had only one drink will give off subtle whiffs of alcohol for 90min or so, at least to anyone who gets physically close to them, hugs them, or is in an enclosed space with them.
Those who see you often, family, wife/husband, bf/gf, close friends might not see the difference because they are exposed to you so often. But if they can tell a positive or negative change in you its usually a good gauge to use to determine what direction you want to pursue in life.
On the contrary, I think that people close to you, who've spend a lot of time around your sober self and know your little quirks and general demeanor pretty well, are more, not less, likely to notice the subtle changes in your affect that a drug will produce.
People who don't know you very well, and have no familiarity with the drug you're on, are likely to just write you and your drug-influenced behavior off as 'weird', unless your behavior is grossly erratic and stereotypical for users of that drug.
Also, keep in mind that people who act different because they're tired, sick, upset, jetlagged, stressed, on a new medication, etc., don't tend to
like the fact that they're not looking, acting, and feeling themselves. Some people get loopy and goofy when they're frazzled, true. But no one gets nonchalantly euphoric! If something about you is "off", and you seem entirely OK with it, that's a big red flag.