it's hard.. but the best that I have done: remember one thing that each differentiates from each other; what do I mean?

PCP--"CRAZY WILD"--have you seen a wild horses? (before they are tamed);
Amphetamines--"paranoid"
Alcohol--Serum GGT: sensitive indicator of alcohol use; emotional
Opoids--constipation, sweat
LSD--"you're beautiful, man...", visual hallucinations
Marijuana--hunger and thirst (increased appetite and dry mouth)
Caffiene and nicotine--can't sit still (restlessness)..

hope that this helps...