if you can monitor the conversation between master and slave, you should be able to see the slave address at the very beginning. A logic analyzer would be more useful since the oscope isn't likely to display the data you need for very long.
the guys at hackaday have something called the bus pirate, it's a usb toy that can monitor various serial buses, i2c included, and dump that data to your PC. It is open source, so you could look at how they're reading the i2c info or just grab one and call it done.
most slave addresses are a static 4 bits as the manufacturer ID and only 2-3 variable bits. So you look up the manuf. id, and then brute force the remaining bits until you hit on something.