There were many porcelain producers in Dresden, Germany that made fine and valuable china. Look for a mark that features a blue crown with five points and three stars, or three points and a central cross. Of course, there are many other marks. You can enter the mark into Google images and see if your particular piece appears. Also, you can sign into Kovels, Replacements, or Worthpoint for information.
Locate a book that can help you identify your plate; "Dresden Porcelain Studios: Identification and Value Guide" by Jim Harren is one. There are other books out there. Check online or at your local library. Remember that older books do not reflect current prices but can help you identify your specific plate.