Ok-I will try and help.....First, silver, particularly serving pieces is usually stterling or plate. Not a combination. Plate means the copper is coated with real silver, whereas Sterling has a higher silver content, and, will not wear off over the years, as plate will. Sterling is usually clearly marked as such, and some pieces may say "925".
If you look on ebay (I have always used them for great info, since they typically have good pics), and do a search for Marshall Fields Silver, it will show you some pieces that are available now, as well as some completed listing, if you choose that option. But, it should give you an idea of the marks you have on your piece, in comparison to others.
If you are ever really trying to ID something-silver, china, crystal, you can always contact www.replacements.com
. They are really good at it. I bought pieces for them for several years.