Initial permeability of toroids in ATX power supply?
The short answer is to measure it. Wind ten turns of wire on the core and measure the inductance. 

There are many ways of measuring the inductance, depending on what test equipment you have available. In a pinch, you can even use sound ports of a desktop or laptop computer. However, you may need to add more turns to the core to get the inductance high enough to work within the frequency range available from the computer.