It depends on the drug and the quantity I've used. I can convincingly feign being sober on alcohol up until about shot number 10 or 12. Moderate recreational amounts of most opiates and opioids make me feel nauseated and weary but sans any exterior indications of being high (excepting miosis); 30-40mg oxycodone makes me queasy, euphoric, and giddy, but I'd seem to be in fine fettle to any onlookers.
I can retain some presence of mind on relatively high doses of most GABAergics (e.g., barbiturates, benzos), but there are some of these drugs that make this more difficult. CNS stimulants (amphetamine, methamphetamine, methylone, cocaine, 4-MAR, etc.) are some of the easiest species of drugs to comport myself unsuspiciously under. If I overdo it, some eyebrows may raise because of my hyperactivity, verbosity, and overall mania. But these signs are nonspecific, and can easily be ascribed to one's personality or chalked up to mere excitement.
In fact, I find a believable affectation of normalcy and self-possession is typically doable for most drugs. The only class of psychoactive drugs for which this is not the case, IME, is hallucinogens. With, say, PCP or shrooms or acid or ketamine, it appears I've only ever taken enough when I think I've taken too much. And when I think I've taken too much, there is no chance I could walk into a crowd and not seem completely out of kilter.