I had a friend who worked in a lab that police/paramedics would send various blood samples to for drug testing.
He said that in a 4 year period, out of all the hundreds of date rape drug test requests, 100% percent of them showed nothing but alcohol.
He said it was a real eye opener for him, how people (in this case women) underestimate the power and simplicity of alcohol. Drinks can be mixed that almost completely mask the taste of booze.
Now I know that this was only in his particular state, and not all states will have the same results, but I still think it's telling over all.
Be less worried about pills getting slipped and more wary of the fact that alcohol can completely drop your inhibitions,