I2C -Multiple Slaves -Common Interrupt -How to Identify Interrupt Source

Multiple I2C slave devices (e.g. 8574 GPIO) share the common interrupt line.
How can Arduino identify which device originated the interrupt?

Read out the state of all connected devices, store them on the Arduino. Once an interrupt is received read the state again and compare to the stored state. You should have a difference on at least on chip. It's quite probable that this chip issued the interrupt.