Maker's Mark
One of the first identifying features of a true antique plate is the maker's mark. The color of the mark, as well as the application of the mark, can give you clues as to whether the piece is an antique or a replica. First, does the mark appear to be under or over the glaze? In a true antique piece, marks were added before the piece was fired, so they are found under the layer of glaze. A replica may have a mark stamped on it that sits on top of the glaze, which sends up an immediate red flag. Also, look for red or blue maker's marks. Before the 19th century, porcelain and china makers only had those two colors that could withstand the heat from firing.
Country of Origin
After 1891 it became a requirement by Congress to place the china's country of origin somewhere on the piece. Some makers placed the country name on places that were hard to find, so search your piece carefully. If you do find a country, that means your piece was likely made after 1891. Keep in mind, however, that some pre-1891 pieces do have marks with a country in the name or within the symbol.
Quality of Glazing
Looking at the quality of glazing can also give you clues about the age of a piece. Remember that long ago, all glazing was done by hand. And in that glazing you'll probably find mistakes, nicks where the clamps held onto the piece, or a pooling of darker glaze in cracks and crevices. If the glaze on your piece is flawless, then there is a good chance it was made by machinery. But if you find general inconsistencies in the piece, you can bet that it was done by hand---a sign that it is antique.