I have documented a very simple liquid tester here:
http://ericholm.nl/blog/2009/08/liquid-tester/
It can measure the resistance of a liquid. Because every liquid has a different resistance you can separate different drinks like water or orange juice.
The cool thing is that you don't need any sensors to build it only a resistor.
When you pass electric current through a liquid, you leave metal ions from the electrods in the liquid, and metal ions can be very toxic. I hope you're not planning to drink any of the liquids after.
O really I didn't know that? do you have more information about it?
How long does it take? so if I hold it for ten seconds in a liquid the liquid becomes toxic? thats a quick way to make a liquid toxic? maybe I have to rename it to 'liquid toxic maker' instead of 'liquid tester'
I think the process is called electrolysis. You can google more information about it (there are serveral types of electrolysis so finding the right one may take a bit) but as Oracle said, the current flow creates ions in the metal conductors and the liquid which it is flowing through, rendering the liquid toxic (how toxic I'm not sure) and sometimes producing toxic gas. I think some liquids are more or less toxic than others. Some might not be toxic at all, but I'm not an expert so don't count on it... 
do you have more information about it?
How long does it take?
I don't have a link in front of me (this goes back to my chemistry classes). The amount of metal ions is a function of how much current is flowing multiplied by the time it's flowing for. How toxic it is depends on what metal the electrodes are made of and how much metal ion you've added. (Lead or Mercury ions are much worse than Gold or Iron for example. Aluminum might be linked to Alzheimer's. Silver turns your skin blue -- permanently).
Also once consumed they generally stay in your body for life and it's a cumulative lifetime exposure that counts.