Debugging I2C bus using an oscilloscope
The easiest thing would be to trigger on the start condition.  That is SDA falling while SCL is high.  This could be detected with a D flip flop clocked by the falling edge of SDA and with SCL as the data input.  The trigger will probably be reset some time during the message since SDA will likely wiggle.  However, in those cases SCL should always be low.  Set the scope to trigger on the rising edge of Q to trigger on the start of a message.