Help please! Caffeine

Hello,

I have a 3 and half month old cat, Jack, who is always finding the wrong stuff to put his nose in, like any kitten would...

Well, I went to the bathroom and when I got back to the living room I caught Jack licking the rim of my (already empty) milk with coffee mug... I know caffeine is toxic for cats, is it possible that the small amount of caffeine in the milk and coffee left on the mug rim might harm him?

What are the signs I should be looking for in case of emergency?

Thank you in advance for any help

Here's a link to signs to look for:  http://www.petpoisonhelpline.com/poison/caffeine/

But if the cup was empty and it was just a drop or two on the rim, that's probably not enough to hurt him.