Sometimes a country makes it easy by putting the year the stamp was issued right on the stamp. Catalog value is NOT what a dealer would pay to buy the stamp from you. The catalog gives a price for each stamp in mint, or unused condition, and also used. These prices are only a general outline of what someone might pay when purchasing the stamp. Scott immediately increases the face value of a new stamp to make it a catalog value when they list an item.
Used stamps vary greatly in catalog price.
Stamp value is related to condition of the stamp and how much demand there is for it. A lot of older U. S. stamps can be bought in packets or lots of 100 or more for a price well below the catalog value. But the catalog value is a good method for the exchange of stamps with other collectors. Every stamp collector uses a stamp catalog. The majority of collectors in the U. S. own a Scott Catalog and may use them as a check list for what they own and need.