reivax

also, there is an object called [bonk~] which is meant to do some kind of recognition, but i have never personally used that feature. you could try though.

however, if you wanted to get more technical, what you could do is record a couple of samples of cans being opened, and run them through spectral analysis to find what frequencies are dominant, and bandpass your audio input so that only these frequencies are allowed through.
and then when all of the dominant frequencies are present, and much higher in amplitude than other frequencies, you'd be pretty sure that a can has just been opened.

so, i reckon if you're doing this as an exersize in programming, then go the latter route, but if you just want practical results, then just get a mic nice and close to the can - and use simple envelope detection.