Tim W. wrote, "Ferrite cores usually have higher permitivity, which
means more
inductance for the same number of winds."

Presumably he means permeability.

You should be able to find both downloadable programs and web applets
which let you find the inductance for a given number of turns, for a
core of particular dimensions and permeability. Using that info, you
can go back to permeability if you measure the dimensions and an
inductance with a known number of turns.

There are also fairly standard color codes for powdered iron toroids,
though, as they say, your mileage may vary. But in general if you find
a permeability below about 50, it's very likely a powdered iron core,
and above that it's much more likely to be ferrite.

You can find some good core info at the Amidon Associates web site, or
at least could the last time I looked. A Google search should turn up
lots of other references.

Cheers,
Tom