The Iron Cross is a confusing decoration in many ways because it was never produced in a single version or by a single manufacturer during both world wars. There is a wide array of versions and maker marks that can be found on the various examples that find their way onto the market.
When buying an Iron Cross keep these facts in mind:
1. ALL Iron Crosses were of a three piece construction. In the case of the 2nd Class the central iron plate was soldered into a guilt edged frame with a suspension ring for the ribbon soldered on top, for the 1st Class the front part of the medal looks the same as the 2nd Class, but the back is a plain, flat plate that has a retaining pin and catch. The central plate should not rattle around or moves inside, it should be set solidly within the frame. You should be able to see a very fine seam around the outer edge of the cross. you should also see a fine separation on the interior of the cross between the iron plate and frame, enough to slide a piece of paper between them. If there is no seam or separation present, the cross is a FAKE, made from a casting, these are the most common fakes on the collector market.
2. Not all Iron Crosses were magnetic. Yes the cross was normally made of iron, but in both world wars, different metals were sometimes used, especially near the end of both conflicts. If your cross does not react to a magnet, it does not mean that its a fake, however it is a good sign.
3. Not all Iron Crosses had a maker mark. It is estimated that up to 20% of the WW1 crosses had no maker mark (I have one myself), this was very common near the end of the war. The percentage is the same for the WW2 version, possibly even higher. On the 2nd class crosses, the maker mark would be located on the ribbon suspension ring, on the 1st class crosses it was normally located on the retaining pin. If your cross does not have a maker mark, again, it does not automatically mean that it is a fake, again however, it is a good sign. though. There are a number of sites online that list the maker marks for both versions of the Iron Cross.
4. The vast majority of Iron Crosses will have some form of patina or aging present. This can be said of any antique, so I'll be more specific. Unless the example was placed in a cool dark place and almost never handled, the cross should show some form of wear. Remember, these decorations (unlike those of most other nations) were worn day-to-day on a soldiers uniform and went into battle with them. Usually there is some wearing away of paint from the edges of the cross, sometimes rust, the guilt edge will also show a patina, normally a grey or brownish color. A sparkly, new looking cross should be treated with caution. Keep the other rules in mind.
Best of Luck