Simply check digital input status of extension in a timer routine. It's not simple as checking a master processor port, but it's the best thing you can do. You will get a byte showing the status of each input.
As I remember PCF8574 has an INT(errupt) output. It's state changes on every change on any of 8 PCF8574 pins set as inputs.
You can connect it to your board's input pin and do the readings according to that change on INT.