[HomeBrew] How to tell between iron core and ferrite toroid?

[Ray, W4BYG]

I have two NOS T-240 size unmarked, uncolored, toroid cores which I 
would like to consider for building a 1KW Guanella balun.

Is there a reasonable way to determine if they are iron core or ferrite?
Ray, W4BYG

-- 
Ironic:
The food stamp program, part of the Department of Agriculture, is pleased to be distributing the greatest amount of food stamps ever.
Meanwhile, the Park Service, also part of the Department of Agriculture, asks us to "please do not feed the animals" because the
animals may grow dependent and not learn to take care of themselves...  (Think about that).