[anyoan1]

What analytical method should you use to see the differences between the two?

Two samples of the same compound - potassium iodide / iodine.
Compounds somehow are diffrent on how they behave.
(I don't have on mind impurities, just molecular thing [maybe magnetic])

[Borek]

How do you know samples behave differently?

Can you tell what is the difference observed? If so, just use method that tests the parameter you know is different.

If you don't know what is the difference you can just blindly tests everything step by step, method by method.

Most parameters should be identical though, they are property of the compound and can't differ at random.