Yes, with a sniffer and some analyzing software is it simple. I2C is a common protocol and SDA and SCL pins are known. Cheap sniffers can be bought. I have built a sniffer consisting of only two components, a 80c51 and a cap for Vcc decoupling. With size 3*3 mm do it fit inside most things. It is clocked at 100 MHz as I2C data rate can be up to 400 kHz, Do not know if it is optimal solution, it was what I had available useable as sniffer. MCU's with existing I2C interface is as far as I know, less suited for sniffing.