You know barn - belikin beer!!

A good bartender(myself included) can pick up an open bottle of both tonic & soda, swirl them one time and tell you which is which. They do it all the time. It is a drama if they are mixed up.
The face of the customer tells it all.
Scotch and Tonic?==yuk

All you wanted to know about tonic and then some:

http://en.wikipedia.org/wiki/Tonic_water

see,, now that's part of the reason i drink belikin.
it's in a dark bottle. you can't see when it's 'turned', you can't see the bugs or ciggy butts, or chicken bones floating in it!
just drink in the goodness! with no worries or concerns.