Excerpt from Transmission Line Transformers (ARRL book #1700)

"For those who have obtained some unknown rods and/or toroids and want 
assistance in identifying them, here are some suggestions:
A) Appearance.  Ferrites can come in all shades of black and gray-black. 
They can be either shiny or dull.  In some cases they can have a 
protective coating (for the wire).  Therefore, low-permeability 
nickel-zinc ferrite is indistinguishable in appearance from 
high-permeability nickel-zinc ferrite or from manganese-zinc ferrite.  
But powdered-iron toroids (which are not recommended because of their 
very low permeabilities) usually have distinctive protective coatings.  
The popular T-200-2 toroid has a clear enamel-like finish and a definite 
undercoating of red.  It has a permeability of only 10 and is called the 
"red mixture."  There are other undercoatings (yellow, blue, and so on) 
for lower-permeability toroids which are to be used at higher frequencies 
for inductors or conventional transformers.
B) Magnet test.  All ferrites and powdered-irons are attracted to magnets. 
Therefore, testing with a permanent magnet is useless.

C) Electrical test.  Toroidal cores can be tested for permeability, and 
hence core material, by an inductance measurement.  The measurement can 
be made directly with an inductance meter or indirectly by some resonant 
circuit.  Either measurement involves the geometry of the core and the 
number of turns used in the winding (see Eq 2-1).  Rod cores, because of 
their large air path, and hence reluctance, don't lend themselves as 
readily to similar measurements.  But all of the rods seen by the author 
at surplus houses and flea markets have been of 125 permeability material 
and are very likely usable.  This material is used in AM radios (loop-stick 
antennas) and is excellent for transmission line transformers.

D) Power test.  For those who are not fortunate enough to have access to 
sophisticated test equipment, another avenue is still available.  It is the 
Soak test described in Sec 12.5.  It involves applying considerable power 
(about 500 W) to a transformer with an unknown core connected back-to-back 
with a known transformer.  One can quickly find out, by the temperature 
rise, if the ferrite is suitable for use.  Ferrites with permeabilities of 
40 to 300 exhibit virtually no temperature rise.  If the toroids and rods 
are small (less than 1 1/2-inch in OD or 3/8-inch in diameter for the 
toroids and rods, respectively), then 200 W would be sufficient.  The author 
has used one of the 12.5:50-Ohm designs described in Sec 6.3 for the test.
E) Generally, all of the low-permeability ferrites from the various
manufacturers can be interchanged without any significant difference
in performance.  Therefore, it is a matter of selecting the right 
permeability range.  The following is a list of codes for equivalent 
ferrites:
1) mu = 40-50: 67, Q2, H54Z
2) mu = 125: 61, 4C4, Q1, H53Z
3) mu = 250-300: 64, 66, K5, H52A, KM1, 250L