How To Know If Someone's Drink Has Been Spiked

How can I know if my someone's drink has been spiked?

If you have any doubt about having left your drink for a long period of time, or you see somebody hanging around your drink, the best thing to do is not to drink from it. If you are with people, get them to watch your drink while you go away from it - maybe to the toilet or something. And if you do think, after drinking from it, that you feel strange, or not as you normally feel, then you should seek emergency medical attention.