It's a very simple three-step process (not really): Step one is freak out. Step two is hold down the power button and turn off your laptop. Step three is take the damn thing apart and dry it out.
If taking your laptop apart sounds like the kinda thing to void a warranty, you're right—it is. But the good (bad?) news is that you've already voided your warranty by spilling your drink on it.
And yes, they can tell. Apple recently implemented spill detection into the unibody MacBooks and MacBook Pros. These ‘Liquid Submersion Detectors’ are small sensors located under the keyboard close to the trackpad and change color if they come into contact with liquid.
So if you're finally ready to suck it up and get down and dirty with the wet inner workings of your MacBook, follow Jackson Chung's guide here at MakeUseOf.com.