Anyone who has watched an episode of the "Antiques Roadshow" is aware of the importance of makers' marks on everything from silver, china to jewelry. If you walk through your home and pick up an item, the mark on it can tell you a lot of information other than if it is Tiffany or Wal-Mart. In 1891 McKinley was President and a tariff act became the law. From this date everything had to have a stamp or a label on it giving its country of origin. So if you are out at the Salvation Army store and you see an old tea cup if it is marked "England" or "France" without the words "made in" you have an old teacup. If it has no country of origin it could be even older. Around 1919 the words "made in" were required. So in order of age no mark= oldest, single country name=older, and made in country name =newer. This is an easy to remember rough guide for dating things in the field.
If you want to do more in depth research, on a mark there are books and web site to refer to. Ebay is useful for more than shopping. If you find something look on ebay a for similar item. Many auctions give great information about what they are selling.
Here are some more date indicators. Any hand written marks tend to be pre-1800. A kite shaped mark with
"RD" in the center dates from 1842-43. A printed or stamped mark in any ink other than blue is post 1850. If the term "LTD" limited is used it is after 1860. The word trademark used after the name of a country of origin dates to after 1891. If an item has a US patent number it can be used to date the item.
It is sad that the mark we see most often these days is Made in China. This mark on an item does nothing to make you think the item is well made. In the future there will not be many vintage items left with Made in China on them they simply will not last that long.
Last week I was out in a local Goodwill store looking for marks on China. The display shelves in this Goodwill are at least 5 feet high containing 3 shelves. I was standing close to a woman who was talking away on her cell phone. She knocked a stack of at least three taped together full size dinner plates off of the highest shelf. They fell and the edge hit my poor foot. It hurt, and I screamed I couldn't help it. The woman did not miss a beat she kept on talking didn't say she was sorry or ask if I was ok. Some other shoppers came to my aid. The woman still blabbing on the phone rolled her buggy out of sight and I limped on.

A grade school Valentine vintage 1950... removed from an old scrapbook.. if you look closely you can see the vintage paste on the front.