The stamp collector's most important weapon is the catalog, which is published every year. The catalog is the collector's reference work. Here you can find all the stamps cataloged and numbered, and here you can also read what your own stamps are worth if they are optimally preserved.
Stamps are sensitive - you should only ever touch them with stamp tweezers. The perforation of the stamps is particularly important: torn perforations immediately reduce the value of each stamp.
But stamps differ not only in that they are canceled or mint never hinged. Some brands look exactly the same at first glance, but the collector can notice clear differences. For example the perforation, which can be found out using a perforation key.
Many brands are also distinguished by the watermark. You can find out by placing the stamp upside down in a small tub of mineral spirits. You can now clearly see the pattern of the watermark. By the way, this also works with mint copies, the gum coating of which is not damaged.
True philatelists even have a color guide with which they can determine the different color nuances of a stamp - there are sometimes big differences there too.
