Tempered marcel irons similar to the old cast iron skillets seem to hold heat more evenly and hold heat much longer before needing to reheat. When comparing the pictures of the two smaller marcel irons, the one on the right is tempered.
Some irons are pre-tempered and ready to use. The brand Golden Supreme usually offers pre-tempered irons. The packaging should say "pre-tempered and ready to use." They cost a little more but are built solid. Kizure brand irons also offer some pre-tempered irons. I actually feel that the shape of the Kizure brand pressing comb is easier to press hair with.
When comparing the pictures of the two large marcel irons, the one on the left is tempered.
How do you temper the non-tempered marcel irons? If my memory serves correctly, then I was taught . . . to heat the non-tempered iron up to "red hot" temperature then dip it into some hair oil or curling wax. Then let it cool down. Then re-heat the iron to "red hot." Then let it cool down again. It should be tempered and ready to use. Watch the stove carefully while you are completing this process; you do not want to start a fire!