Stoners, esp. long term users drag out words (talk like they're perma-stoned). Red eyes, usually munching on fast food ravenously by 4:40.
Coke/pills insufflated ROA: Many use the bathroom too many times...never flush toilet or wash hands yet leave the bathroom noticably relieved even though they have suddenly developed the sniffles.
Pupils are huge or tiny...dark circles under eyes and poor skin tone...possibly acne.
Sudden rapid weight loss and new physical, psychological, and monetary complaints.
Hallucinogens...the pupils and the grin usually give it away.
Opiates...the scratching...
And no, i wouldn't approach or ask anyone i didn't know just for a hookup...but i've used up my one and only 'get out of felony drug charges' chance by successfully completing Drug Court and i wouldn't take any risk of getting caught after that.