I mean about if I had a piece of metal, how can i decide that's iron or steel?Sorry for bad English.
Iron is a chemical element with the symbol Fe, that has no engineering application as it is very soft. You are not likely to come across a piece of metal of pure Iron. Steel is an alloy of Iron that is widely used with different types for various application. The type depends on the alloying elements. An old and rough way of identifying them is to use a chart, that shows the spark pattern when they are subjected to grinding.
Iron and steel have different thermal expansions and specific heats. You could either heat it up and see how much it changes in length or you could put it in water inside a calorimeter and observe how much the temperature of the water changes
There are literally hundreds and hundreds of different commercial steel alloys and a good number of iron alloys so... it depends. Technically, steel is any alloy that is mostly Fe with some amount of carbon. Even if you have access to the necessary lab equipment, CTE (coef of thermal expansion) and specific heat are not very straight forward tests. There are, of course, chemical analysis instruments for measuring C content in steels so, if you really need to know, you can spend money at commercial lab that does these tests. One quick test to judge C content... touch the metal to a grinding wheel, higher C content produces more sparks. To calibrate the way this looks, compare the sparks from a drill bit (usually 1090 steel so 0.9 wt% C) to a steel nail (low C steel). Unfortunately, the difference in sparking between steels with pretty low C content and a piece of iron will not be very dramatic. Of course, iron might mean Cast Iron and, again, there are a good number of commercial cast iron alloys and there are 4 basic types of cast iron alloys and they contain high amounts of C.
