You're in a club and you see a girl leave her drink at the
bar when she goes to powder her nose. When she comes back, instead of actually spiking her drink, rush over to her and scream "
DON'T DRINK THAT!". Explain you saw some middle-aged
guy slip something into her drink and you just wanted to make sure she didn't end up Fritzl'd.
Next, slyly suggest she needs a new drink and that you would be willing to buy it her. How it goes from there is up to you.