The only scope I have access to is at work. I did put a couple of LEDs across the lines and GND. When it's unused, both LEDs illuminate dimly (of course it could be flashing very fast) when I send a request out the LED on the SDA line goes out and flashes briefly. Looking at the datasheet for the PCF8574 the start condition indicates the SDA line goes low to signify the start of the session. The SCL line doesn't change. Once the program has finished the SDA LED lights up again.