First thing to remember is that both "true" cinnamon and cassia come from the same genus, which contains about a dozen or so species, not all of which are used as a culinary spice, so when talking about "true" cinnamon vs. fake cinnamon, or cassia, the differences are not huge. What is called "true" cinnamon comes from Ceylon. Since it usually garners a higher price it is usually referred to either Ceylon or Sri Lanka cinnamon to differentiate it from other cinnamons (aka cassia). When ground it is pretty much impossible to tell the difference by looks alone. By aroma, it is less pungent than cassia, with notes of citrus, but again it can be difficult to tell, if not almost impossible, especially for someone not readily familiar with the differences. In stick form it is pretty easy to tell as cassia is made up of 1 thick layer of bark rolled into a quill, while "true" cinnamon has many delicate layers.
Back to the health issue. It seems that cassia contains a significantly higher amount of coumarin (sp?) which is a natural blood thinner, and as such taking cassia in high doses can cause liver damage. This really doesn't seem to be an issue for people who just eat cinnamon in regularly quantities, but for those that take cinnamon supplements or oils to treat health conditions. One of the issues is the supplement industry, which relies on herbs and spices, is not regulated by the FDA to the extent that drugs are, and in fact the FDA does not require that a company prove that their supplements are safe. People need to stop thinking that just because it's "natural," that it's an herb or spice, that taking a supplement is safe. As I stated before, many herbs and spices do have medical properties beyond just culinary properties, but taking them in excess, just like taking any drug can be toxic. Before taking any supplement one should always consult with one's doctor to make sure that they are not harming themselves, and then purchase supplements only from reputable suppliers. Just recently a study was done, testing supplements and many of them not only, didn't contain significant amounts of the herb that was supposed to be in them, but contained all sorts of other herbs not listed.-end of my PSA.