To determine the year of issue, you really need to have access to a set of Scott catalogs. The catalogs don't have to be the latest edition. You can buy a five year-old set of catalogs for about $40. Also most town public libraries have them. Just keep in mind the the newest stamps won't be listed, but many of the new stamps have the date printed on them anyway.
You can often get a clue from the denomination. The majority of stamps are valued at the first class letter rate. As you page through the catalog listings for a country, you can see how the first class letter rate increases over the years. Since postal rates increase every few years, you can narrow your focus down quickly if it is a first class stamp.
The second clue is the style of the design. Stamps from the 19th century usually have frames and elaborate engraved designs. As you move through the different decades you can see how the styles and colors change. In the twentieth century there are less and less engraved stamps and more printed by lithography and other modern printing technologies. By the 1960s the designs have much less formality and begin to resemble poster art. After a while you will be able to recognize the "era" of the design right off and narrow your search to within a decade or so.
The third clue is the subject of the stamp. For example, if Queen Victoria is on the stamp, chances are good that it was issued during her reign (before 1901) or shortly after her death. If Albert Einstein is on the stamp, chances are it was issued sometime after his death in 1955.
With some practice, you will be able to locate a stamp in the catalog very quickly.